Hydrogen and oxygen are often found in the form of H2O but even if hydrogen surpasses oxygène in number it doesn't when it comes to mass because M(O) = approximately x16 M(H)
It is given that the allele B for the black coat is dominant over allele b for a brown coat.
A dominant allele always leads to the expression of the dominant trait in the heterozygous condition.
When a black and brown horse is crossed as given in the question and the result is a brown foal, then the genotype of the brown foal must be bb as the brown coat is a recessive trait.
Since each parent contributes one allele each for a given gene, the black horse must have b allele, and hence must be heterozygous Bb.
Any radiation of any kind can cause cancer. So many things today can be connected to cancer that making the assumption that radiation for cervical cancer causes liver cancer is plausible.