如图所示，杆BC的B端铰接在竖直墙上，另一端C为一滑轮。重物G上系一绳经过滑轮固定于墙上A点处，杆恰好平衡。若将绳的A端沿墙向下移，再使之平衡（BC杆、滑轮、绳的质量及摩擦均不计），则

A.绳的拉力增大，BC杆受压力增大    

B.绳的拉力不变，BC杆受压力增大

C.绳的拉力不变，BC杆受压力减小    

D.绳的拉力不变，BC杆受压力不变

School failure and social isolation can lead to early onset of depression in children and teens, and the trend is becoming increasingly common.