如图,将长方形纸片先沿虚线AB向右对折,接着将对折后的纸片沿虚线CD向下对折,然后剪下一个小三角形,再将纸片打开,那么打开后的展开图是( )
A:
B:
C:
D:
对于全集U的子集M,N,若M是N的真子集,则下列集合中必为空集的是( )
⑴____A. morning B. good C. afternoon
⑵____A. friend B. sister C. brother
⑶____A. this B. I C. you
⑷____A. is B. am C. be
⑸____A. he B. she C. my