Ummm....
OP (or rather GP) said "Cacti is a plural of cactus."
You replied with "Cactus is (also) an English word and thus its plural is cactuses."
Only one of you made an assertion that implied that there was only one correct answer.
OP (or rather GP) said "Cacti is a plural of cactus."
You replied with "Cactus is (also) an English word and thus its plural is cactuses."
Only one of you made an assertion that implied that there was only one correct answer.