_______she is not so cheerful________she used to be?

A．How it is that；as

B．How is it that；as

C．Is it why；that

D．Why is it that；what

以丙酮、甲醇为原料，以氢氰酸。硫酸为主要试剂合成a—甲基丙烯酸甲酯。