Can you ride ______bike?—Yes. What about going to the park by bike this coming Saturday?

Mom,where is David?—He is working _____ that difficult math problem in his room.[ ]

This is ______ book I’ve told you about. It’s written by ______ university teacher.

Please be quick! The meeting will start ________.

My parents always ________ the best in me and encourage（鼓励） me.