Read the following statements carefully and choose the correct statements:
a. In a transcription unit, the promoter is located at the $5'$ end of the coding strand.
b. The single-stranded DNA having the polarity $5'\rightarrow3'$ is the template strand.
c. RNA polymerase binds to the operator during transcription.
d. Single base DNA differences in humans are called Single Nucleotide Polymorphisms (SNPs).