20．_______ has recently been done to provide more buses for people, a shortage of public vehicles remains a serious problem.

A．That　　 　　　　　　　 B．What　 　　　　　　　　 C．In spite of what　 　　 D．Though wat

19．_______, I have to put it away and focus my attention on study this weak.

A．However the story is amusing　　 　　　　　　 B．No matter amusing the story is

C．However amusing the story is　　 　　　　　　　 D．No matter how the story is amusing

18．It was not until dark ______ he found ______ he thought was the correct way to solve the problem.

A．when; what　 　　　 B．when; that　 　　　　 C．that; what　 　　　　　 D．that; that