13. Which of the following best describes thymine dimers? (A) intrastrand crosslinks induced by UV radiation (B) most commonly produced through oxidative damage to DNA, which is caused by the by-products of mitochondrial respiration (C) recognized by MutS and excised by MutH, when MutH is activated by MutL (D) byproducts of DNA replication, formed when one dTTP molecule is mistakenly linked to another rather than to the 3' end of the growing strand