Which Question(s) Must Be Asked When Calculating a Heritability Ratio

Which question(s) must be asked when calculating a heritability ratio?

A) How much phenotype variance is there in the group?
B) How much of the phenotypic variance in a group is caused by differences in the genome?
C) How much of the genotypic variance in a group is caused by differences in the genome?
D) Both a and b, but not c.