_______ excited his parents most is that he had passed the entrance exam and was admitted into his dream college.

A．That

B．What

C．Which

D．It

根毛细胞的结构特点是（）。

A.细胞壁厚，细胞质少，液泡很大

B.细胞壁薄.细胞质少，液泡很大

C.细胞壁厚，细胞质多，液泡碎小

D.细胞壁薄，细胞质多，液泡碎小