1.      People used to think that the earth was square

@  The earth …………………………………………………………………

2.      My advice to you is to study harder next semester .

@ If I ……………………………………………...........……………………

3.      The beach was so crowded that there was hardly room to sit down.

@  There were ………………………………………………………………

4.       No explanation is necessary.

@ It is ……………………………………………………………….............