—The guard keeps         watch at the gate.

—So he does. Only those         knows well could be let in.

A．close; who

B．tight; whom

C．tight; that

D．close; he

用HCl滴定液滴定氨溶液时，应使用

A．酚酞

B．甲基橙

C．甲基红

D．溴甲酚绿

E．百里酚酞