关于速度和加速度的关系，下列说法中正确的是（   ）

如图，DE与$△ABC$的边AB,AC分别相交于D,E两点，且DE//BC．若AD：BD=3：1, DE=6，则BC等于（      ）．

    Mike is a factory worker. He is often very tired after a day's work. His wife, Jenny, has no job, so she stays at home to cook the meals. Every day he can have his dinner when he gets home from his factory.  

    One day, Mike came home very late because he was very busy in the factory. He was very hungry when he got home. He was not happy when he found his dinner was not ready. He was very angry with his wife. He shouted at her, "I'm going out to eat in a restaurant." "Wait for five minutes，" said his wife. "Why? Do you think that dinner will be ready in five minutes?" asked Mike. "Of course not," she answered. "But I can be ready to go with you in five minutes."