which or what

[Julierose]

"How many vowels are there in the english alphabet. Which are they?"
OR
"How many vowels are there in the english alphabet. What are they?"

Could somebody please explain?

Thanks.

[emsr2d2]

1) There are 26 letters in the English alphabet. The alphabet consists of A B C D E F G H I J K L M N O P Q R S T U V W X Y and Z. Which are the vowels?

2) In the English alphabet, what are the vowels?

In the first, you have been given a list of options to choose from (the letters from A to Z) so the question is "Which...?"

In the second, it's a straight question with no options given so the question is "What are the vowels in the English alphabet?"