The library is ( ) the first floor. 括号里该填什么

Question

选项有A.at B. in C.on D.under
first floor是一楼的意思吧？

Answer 1

C选项，一般都是on ... floor

Answer 2

答案先C. on

Answer 3

答案选C在第2层楼上

Answer 4

C on the floor ,在几层上面，On 有表示在什么上面的意思