In Bangkok severe flooding, many major roads ________ north, towards the areas worst affected

by the flooding, were largely impassable. [      ]

A. to lead          

B. leading          

C. led            

D. to be led

HGPRT参与的反应是

A．嘌呤核苷酸从头合成
B．嘧啶核苷酸从头合成
C．嘌呤核苷酸补救合成
D．嘧啶核苷酸补救合成
E．嘌呤核苷酸分解代谢