百日咳是通过下列哪种方式传播：（）
A.直接接触传播
B.间接接触传播
C.虫媒传播
D.空气飞沫传播
参考答案：D
One of the difficulties we have _____ English is how to remember new words and expressions. [ ]
A. to learn
B. in learning
C. learn
D. learned
用重结晶法分离提纯药物或中间体时，对溶剂的选择有哪些要求？
