—How does Jack usually go to school?

--He_____a bike, but now he_____there to keep healthy.

A．used to riding, is used to walk

B．was used to riding, is used to walking

C．used to ride, is used to walking

D．used to ride, is used to walk

在盛水的烧杯里放一木块，水没有从杯内溢出，则水对杯底的压强与不放木块相比较（ ）

A．压强不变

B．压强增大

C．压强减小

D．无法判定