Jenny says she might ______ a doctor in the future. [ ]
A. is
B. be
C. makes
D. to be
答案:B
茶壶、锅炉水位计和船闸等都是利用______原理工作的;气体打火机燃烧用的丁烷气体,是用______的办法使它成为液态,贮存在打火机里的.
划线盘是在工件上直接划线或找正工件位置的工具。( )