Cloning Vectors - Medical Science

Q1:

A plasmid can be considered as a suitable cloning vector if

A it can be readily isolated from the cells

B it possesses a single restriction site for one or more restriction enzymes

C insertion of foreign DNA does not alter its replication properties

D All of the above

ANS:D - All of the above

How option B?

According to above one restriction site is available for one restriction enzyme. If it is correct then why we need the concept of more fragmentation if we increases the time.