Bill is ill, ___ he doesn't come to school today.[ ]
A. also
B. and
C. but
D. because
答案:B
在某介质中形成一列简谐横波,该横波上有相距4m的A、B两点,下图所示为A、B两质点的振动图象,若这列波的波长大于2m,求:这列波的波速.
CRH2A型动车组绝缘测量的主要项目和标准?