What are primers?

A、Primers are the short sequences at the end of the nucleotide sequences which are used for amplification;

B、Primers are the short sequences which are complementary to the nucleotides at the end of the sequence which is to be amplified;

C、Primers are the short sequences present anywhere in the nucleotide sequence to be amplified;

D、Primers are the short sequences which are complementary to the nucleotides anywhere in the sequence to be amplified