Mr. Jones teaches history. He is a very good teacher. During this year, most of his students got at least a C on the tests and there were many A's. On the last test, however, Mr. Jones got a big surprise. He wrote down all of his students' scores. Then he counted the number of students that got between 91 and 100. There were only two! Only two students got A's! He found out that there were only a few B's and C's. Most of the students in his class got a D or and F on the final test. Mr. Jones looked at the test again. He thought that it was too hard. He decided to give every student 10 more points. Then many of the students got good grades on the test. What a nice teacher!

黄河中下游含沙量大的重要原因是（    ）  

质量为0.5千克的小球被抛出去，如果不计空气阻力，则小球受到 ____力的作用，该力的大小是　 ____，方向是____，施力物体为 ____