根据短文，选择正确答案。

    This is a picture of a family. The grandmother's name is Harry Smith. The grandfather's name is Jean Smith. They are very old. They are Americans.

    The father'sname is Lake Smith. He is 44. The mother's name is Kate Smith. She is 40. They have a son of ten and a daughter of fifteen.   

    The son's name is John Smith and the daughter's name is Mary Smith. They are students of No.12 Middle School. Kate is a teacher of the same school. Lake is a policeman. It's a very good family.

“树怕伤皮”，如果将树皮剥去，植物就会死亡，主要是因为破坏树皮中的哪一组织？（　　）

如图所示，在一水平地面上，木箱重500N，用F=150N的力拉动木箱做匀速直线运动（不计滑轮重量及绳与滑轮间的摩擦），木箱受到的摩擦力为　____ N，方向向____ ．