Why Are Two "Hits" Required to Inactivate Tumor-Suppressor Genes

Why are two "hits" required to inactivate tumor-suppressor genes?

A) Each allele must be altered, and each person has two copies, or alleles, of each gene, one from each parent.
B) The first hit stops tissue growth, and the second hit is needed to cause abnormal tissue growth.
C) Tumor-suppressor genes are larger than proto-oncogenes, requiring two hits to affect carcinogenesis.
D) The first hit is insufficient to cause enough damage to cause a mutation.