5．下列物质属于苯的同系物的是( )
答案：A
当苯中的氢原子由烷基代替时可得到苯的同系物，故只有A项符合题意
It is better to travel 10,000 miles than to read 10,000 books, so it’s important to have real ________ with languages, cultures and societies.
A. communication B. experience C, knowledge D. influence
65岁以上单纯主动脉瓣狭窄的常见病因是
A．风湿性瓣膜病
B．单纯主动脉瓣狭窄
C．二叶式主动脉瓣钙化
D．退行性钙化性主动脉瓣狭窄
E．冠状动脉粥样硬化性心脏病
