题目

Last year, a report by a committee of education experts said that a lot of American students cannot write well. The report noted the concerns of business leaders and teachers. The experts said that more students should have to pass a writing test before they can finish high school. They pointed out that major college entrance tests are changing now to include a writing part.Educators know that teaching students to write well is not easy. One problem is the amount of time needed to read through large amounts of work. So some companies have developed computer programs. These can grade student wr在核反应方程的括弧中，X所代表的粒子是 （   ）A.        B.        C.        D.