视网膜黄斑的中央凹含有（）。
A.视杆细胞
B.视锥细胞
C.节细胞
D.色素上皮
E.双极细胞
参考答案：B, D
___was known to all that William had broken his promise ____he would give them a rise [ ]
A. As; which
B. What; that
C. It; that
D. It; which
在相同压力下，氮气的沸点比氧气（）。
A.相同
B.高
C.低
