Learn English – Using “key to” vs “keys to” for a single key

Earphones/earbuds are those worn in the ears without a headband. While they come in pairs, one can have an earphone/earbud in one ear, leaving the other dangling.

Some, but not all, headphones have a band. The earphones or earcups (you may not find this in the dictionary) that make up the pair are worn over or on the ears. Thus, one can wear headphones with only one earcup/earphone on.

Your example could be correctly written in a number of equivalent ways:

• She wore an earphone in one ear and listened to him with the other.
• She had headphones on with an earcup slid off so she could also listen to him.
• She left an earbud in one ear and listened to him with the other.
• She wore headphones but had only one ear covered while she listened to him with the other.
• She wore headphones with only one earcup on, and listened to him with the other ear.
• She wore headphones with a cup over one ear and listened to him with the other.

Once the word earphone is used, it is implied that it is present in a pair or set of earphones/headphones, with only a very few exceptions.

You need to define your terms, which is nearly equivalent to asking whether your friend is a maths nerd.

In maths, many is defined as (relating to) any counting number over 1, so, for instance, the inverse of the squaring function is a one-to-many relation, 4 mapping to both 2 and -2, and the fact that there are two images necessitating the term 'many'.

However, this is not the accepted everyday sense of the word many, and few mathematicians even would use it this way outside their studies. Peter Shor's comment gives the sensible everyday range of usage. And your answer 'B' is the only sensible one given the context.