a change in bowel habits prompts a 53-year-old woman to see her physician. on physical examination there are no lesions noted on digital rectal examination, but her stool is positive for occult blood. a colonoscopy is performed and reveals a 6 cm friable mass inthe cecum. a biopsy of this mass is performed and microscopic examination shows a moderately differentiated adenocarcinoma. after resection of the tumor, the oncologist begins treatment with 5-fluorouracil (5-fu) combined with other chemotherapeutic agents. 5-fu not only kills tumor cells but also rapidly growing cells of the immune system and intestine. what is the mechanism by which 5-fu selectively kills fast growing cells? group of answer choices binds tightly to thymidylate synthase inhibiting the synthesis of thymidine binds tightly to rna polymerase ii shutting down transcription of mrna binds tightly to dna polymerase shutting down replication intercalates between the two strands of dna preventing replication binds tightly to rna polymerase i shutting down synthesis of rrna