如图所示,a、b、c为电场中同一条电场线上的三点,b为ac中点。a、c两点的电势分别为φa=4V、φc=2V。下列说法中正确的是( ) A.该电场中b点的电势一定为3 V B.a点的电场强度一定大于b点的电场强度 C.若正电荷在这个电场中只受电场力作用,且它运动过程中经过a点,它就一定会沿电场线运动到c点 D.正电荷从a点运动到c点,电势能一定减小 She knew nothing about his journey _______ he was likely to be away for three months. A. Except B. except for C. except that D. in addition