下列哪项是导致肺栓塞的最常见病因（）
A.空气
B.脂肪
C.瘤栓
D.羊水
E.血栓
参考答案：E
— Have you heard that some bad thing would happen in 2012?
— Don't worry! The news _____ be true.[ ]
A. mustn't
B. may not
C. needn't
蔬菜原料应分别密封保存防止挥发（）。
A.营养
B.气体
C.油分
D.盐分
