经分解代谢可产生一碳单位的氨基酸是（）
A.谷氨酸
B.酪氨酸
C.苏氨酸
D.组氨酸
E.丙氨酸
参考答案：D
“老吾老以及人之老，幼吾幼以及人之幼。”这句古语体现了善良应该是（ ）
A．是非分明
B．无私的
C．勇敢的
D．博大的
— I had thought he would be nervous in this speech competition, but he turned out to be calm.
— He _____ quite enough preparation for it. [ ]
A. should do
B. must do
C. should have done
D. must have done
