— It seems that Mr. Wu is standing there.— It ______ be him because he called me from Canada just now.
A:mustn't
B:can't
C:needn't
D:shouldn't
Look! Sonia looks worried. There must be ________ wrong with her.
设集合M=x|x=kπ2+π4,k∈Z,N=x|x=kπ±π4,k∈Z,则M、N的关系是( )