某辆汽车在公路上行驶，它行驶的路程s（m）和时间t（s）之间的关系为：s=10t+3t²，那么行驶200m需要______s．

Which of the following statements is true according to what you hear

A．Prices for houses in big cities have started to rise since this year.

B．People buy and sell houses to improve their living conditions.

C．Economists believe that residence prices would drop.

D．Economists warn people that price rise might slow down.