—It's difficult for me ________ there are few new words in it.
A:if
B:so
C:because
D:although
如图,一带负电荷的油滴在匀强电场中运动,其轨迹在竖直平面(纸面)内,且相对于过轨迹最低点P的竖直线对称.忽略空气阻力.由此可知( )