The differences between “中间” and “当中”

The correct answer is A.

Translation:

         Zhèr yǒu sānge píngguǒ, nǐ yào nǎ yíge? Tom:这儿 有    三个    苹果,   你 要  哪 一个?          There are three apples. Which one do you want?

         Wǒ yào zhōngjiān de nà yíge. Jack:我  要      中间      的 那 一个。           I want …