题目

设为非零向量，则“”是“方向相同”的A. 充分而不必要条件 B. 必要而不充分条件C. 充分必要条件 D. 既不充分也不必要条件 答案：【答案】B【解析】根据向量的共线的充要条件，即可作出判定，得到答案．因为为非零向量，所以时，方向相同或相反，因此“”是“方向相同”的必要而不充分条件.故选B．Trees are useful to man in three very important ways. The first important way is that they provide man with food, wood and other products. Trees provide not only man with food, but also many animals with food. Without trees many animals could not live on the earth. It’s not easy for man to live on the earth, either. The second important way is that trees give us shade. On a hot summer day, people are eager(渴望)to have a rest under the shade of a tree after they have walked a long way. You can imagine how important the shade of a tree is to man and to animals. The third important way is that trees help to prevent drought and foods. However, in many parts of the world, man has not realized the third important way. He has cut trees down in large numbers. In the end he finds that he has lost the best friends he had.                          The Importance of Trees The First Important Way: 1.____________________ _________________________    The Second Important Way: To give man shade The Third Important Way: 2.______________________ _________________________ Without trees it’s difficult for man and many animals to live on the earth.        On a hot summer, people 3.________________ _______________________ are eager to have a rest under the shade. 　   Man has not realized this. Because of 4.____________ __________________________ He finds that he has lost the best friends he had