A:There is nothing that we can do to save these trees.
B:There isn't anything that we can do to save these trees.
C:There is nothing that we can do to protect these trees.
已知|a→|=2,|b→|=3,向量a→与b→的夹角为150°,则a→与b→方向的投影为( )
某同学用多用电表按如图所示电路进行了正确的测量。闭合开关S后,多用电表直接测出的是( )
电灯L1和L2并联接入电路,通过它们的电流分别为I1和I2,干路中的电流为I,则 ()