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 sufficient damage to cause a mutation