问题
选择题
Mr. Black ______ Shanghai in a few days. Do you know when the earliest plane ______
on Sunday? [ ]
A. is leaving, takes off
B. leaves, takes off
C. is leaving, is taking off
D. leaves, is taking off
答案
答案：A
；若场区仅存在垂直于xoy平面的匀强磁场时，质子也能通过P点。不计质子的重力。设上述匀强电场的电场强度大小为E、匀强磁场的磁感应强度大小为B，则下列说法中正确的是
