Mr. Smith has bought a new house in the country, around _____ some green trees.

A．which is

B．it is

C．which are

D．them are 

关于脱氧核糖核苷酸的生成过程，错误的说法是

A．由5′-磷酸核糖从头合成
B．由脱氧腺苷生成dAMP
C．脱氧核糖核苷酸的生成都在二磷酸水平
D．GTP生成dGTP
E．由TMP生成dTMP