Which one is the correct answer?

[Daisy]

Children _____ have better memories than adults, especially in learning foreign languages.

A. on average
B. on the average
C. on an average
D. each of

This is a multiple-choice question. I chose A at my first thought, which is right according to the given standard answer. But when I looked up in the dictionary, I found that A, B and C basically have the same meaning. I'm quite confused. Would you please help me in this?

[Alan]

Hi Daisy,

If you want to say something like: generally speaking/by and large/in general or something along those lines, you would use on average. If you use the other expressions: on the average/on an average, you would have to refer specifically to some examples or numbers as in: on an/the average of six times a year or four hours a week and so on.
Hope this helps.

Alan
_________________
English as a Second Language
You can read my ESL story Phrasal Verbs/look