The children _______ a PE class on the playground when it suddenly began to rain.

A．have

B．are having

C．had

D．were having

用下述哪种单纯固定液固定的组织宜避光保存，以防止蛋白质溶解（）

A.重铬酸钾

B.铬酸

C.苦味酸

D.醋酸

E.升汞