如图所示，质量是m=10g的铜导线ab放在光滑的宽度为0.5m的金属滑轨上，滑轨平面与水平面倾角为30°，ab静止时通过电流为10A，要使ab静止，磁感强度至少等于______，方向为______．（取g=10m/s²）

— Why are you still busy with your homework? You _____ it hours ago. 

— Sorry, dad. I haven't been feeling well since early evening. [ ]

A. should have finished

B. should not finish

C. should finish

D. must have finished