The farmer was very thankful to the doctor ______ treated his son.

A．what

B．which

C．who

D．whose

关于tRNA的叙述正确的是

A．分子上的核苷酸序列全部是三联体密码
B．是核糖体组成的一部分
C．可贮存遗传信息
D．由稀有碱基构成发夹结构
E．二级结构为三叶草形