Dalin plays basketball very well. He ______ basketball.
A:is good for
B:is good at
C:good for
D:good at
先化简,再求值:2m+1﹣m-2m2-1÷(1﹣1m2-2m+1).其中m满足一元二次方程m2+(53tan30°)m﹣12cos60°=0.
下列物质由离子直接构成的是( )