The OP's sentence appears to be adapted from a Ph.D project
The etiology of schizophrenia is highly complex, involving both genetic and environmental risk factors in multiple combinations. Among the environmental risk factors, drug abuse in general, and cannabis consumption in particular, play a key role.
Summarised:
Drug abuse and cannabis consumption are just two of the risk factors which may lead to schizophrenia.
The verb are is plural because the author lists two separate risk factors.
But in the OP's example there is no context, there is no premise, we only have the bare statement to go on. The fragment phrases “in general”, and “in particular”, will be omitted for reasons of clarity and because they are not essential to the sentence's construction.
- Why drug abuse, and cannabis consumption, is dangerous for your health.
In this sentence, the two elements ‘drug abuse’ and ‘cannabis’ are singular nouns. Taken individually, the clauses ‘drug abuse are ...’ and ‘the consumption of cannabis are ...’ would be ungrammatical. On the other hand, many native speakers instinctively opt for the single verb when it agrees with the closest noun or noun phrase in that situation. For example,
A. Fresh fruit and organic rice is healthy.
A. Exercise in general, and a diet of fresh fruit and vegetables is good for your health.
The verb is normally singular when two subjects in a clause refer to the same person or thing,
e.g. Bread and butter is the only food he eats.
Similar examples:
B. Fish and chips is a British institution.
Joanna’s rum and coke was equal parts rum and coke. source
But
C. Lack of exercise, and eating fatty foods are bad for your health.
In speech, I strongly doubt anyone would notice or point out the verb disagreement in sentence No.1. However, in writing, and when a certain formality is required, I would recommend that learners use the plural verb when two separate subjects are joined together by and.
- Why drug abuse, and cannabis consumption are dangerous for your health
If we look back to the original statement, the verb play is plural, not singular.
The following examples show that authors usually prefer the plural verb in similar constructions.
Throughout non-communist Southeast Asia communism in general and China in particular have long been the ubiquitous targets of threat perceptions which in very recent years have accorded Vietnam and even the Soviet Union... source
The evidence we marshalled at the time was, frankly, a bit thin. However, time, and this conference in particular, have replicated this finding.* source
The Netherlands and Germany in particular have become places where, within a relatively short period, a host of African-initiated churches have been founded. source
The point is that even when the subect is clearly more than one, using the singular verb is relatively common in speech if the last subject is singular or is perceived to be; but in formal writing, it is always better to use the plural verb when two subjects are joined by "and" although there are a few exceptions, which I have listed above.
References:
- Understanding subject verb agreement
- What Is a Compound Subject?
- Compound Subjects “Milk and cookies”: Does the “and” make it plural?
- 20 Rules of Subject Verb Agreement
Best Answer
This questions falls more under the scope of test question writing than grammar. By convention, when asking multiple-choice questions like this with singular answers and an 'all of the above' option, you phrase your question in the singular. This is because when you select 'all of the above', you're saying that all the other answers can be a correct answer to the question individually. Using the singular what is the benefit, picking 'all of the above' is equivalent to picking all of these:
So even though there are multiple correct answers (and thus using what are the benefits may seem correct at first) in the context of a multiple-choice question I believe this to be best.
There's also the element that if you write the question as what are the benefits you run the risk of tipping off your question takers into selecting 'all of the above', as none of A, B and C work grammatically with the question, leaving D as the only logical choice.