B

    It's 14:00. The house is quiet(安静的). "Tom?" Mom says. No one answers.

    Mom comes to Tom's house. He is not in the bed. But his model plane is on the desk. "He is not in his room, and he is not in the yard(院子)," Mom thinks. Then Mom comes to his grandparents' room. Tom likes to play with Grandpa's tape player. But he is not there.

    Mom looks for(寻找) him everywhere. But where is Tom? Mom is worried. She wants to call Dad. She comes to the living room to get the phone.

    "What's that?" Mom sees something(某物) black under the sofa. Oh, it's Paul, Tom's dog. It loves sleeping(睡觉) there. But what is next to Paul? It's Tom!

立定跳远时，如果起跳板上有沙粒，起跳时就会感觉很滑，影响跳远成绩，这是因为起跳板上的沙粒把滑动变为____，使摩擦力大大减小。

读“甲、乙、丙三地自然景观图”，回答下列小题。