下列哪种疾病咳嗽但不伴随杵状指(趾)
A．支气管扩张症
B．大叶性肺炎
C．慢性肺脓肿
D．慢性脓胸
E．支气管肺癌
参考答案：B
Can 16-year-old students drive to school?
No, they shouldn’t ______ to drive because they aren’t serious enough.
A．allow
B．be allowed
C．allowed
D．have allowed
设二维随机变量(X，Y)的概率密度为
计算E(X)，D(X)；
