—  Where did you find the story book? 

—  It was in the bookstore near my house _________ I often spend my Saturday mornings. [      ]

A. that    

B. where  

C. when    

D. which

如图，一竖直杆固定在小车上，杆顶处用一细绳挂一质量为m的小球，当小车向右加速运动时，细绳与竖直杆的夹角为θ，求细绳的拉力和小车的加速度．