题目

   When people hear a president speak, they seldom think about others helping to shape the presentation. Today, however, presidents depend on writers such as J. Terry Edmonds to help them communicate effectively. Edmonds is the first African American ever to work as a full-time speechwriter for a U.S. president; he is also the first African American to serve as director of speechwriting for the White House. His is an all-American story of success.    Edmonds grew up in Baltimore, Maryland; his father drove a truck, and his mother worked as a waitress. A great reader, Edmonds showed a如图所示，水平放置的两平行金属板间有一竖直方向匀强电场，板长为L，板间距离为d，在距极板右端L处有一竖直放置的屏M，一带电量为q，质量为m的带电质点从两板中央平行于极板射入电场，最后垂直打在M屏上。以下说法中正确的是A．质点打在屏的中央P点上方，板间场强大小为2mg/qB．质点打在屏的中央P点处，板间场强大小为mg/qC．质点打在屏的中央P点下方，板间场强大小为2mg/qD．质点打在屏的中央P点下方，板间场强大小为mg/q