Bacteria are sometimes()too small to be seen without a microphone.

单项选择题My pain()apparent the moment I walked into the room, for the first man I met asked sympathetically: Are you feeling all right

A. must be
B. must have been
C. had been
D. had to be

单项选择题()is known to the world, Beethoven is a great musician.

A.That
B.Which
C.As
D.It