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

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

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