减慢心率从而降低心肌氧耗量，缓解心绞痛的药物是（）
A.硝酸异山梨醇酯
B.硝苯地平
C.胺碘酮
D.美托洛尔
E.硝酸甘油
参考答案：D
被子植物的六大器官是由根、______、叶、花、果实、______组成．
In today's class, the students were asked to () their mistakes on the exam paper and put in their possible corrections.
A．cancel
B．omit
C．extinguish
D．erase
