. --- Steve has got the first prize in the maths contest.

--- ______ is no wonder that he looks so happy today.

A．It

B．That

C．This

D．There

嘌呤核苷酸从头合成的原料是（）

A.天冬氨酸

B.甲硫氨酸

C.丝氨酸

D.酪氨酸

E.精氨酸