Какон проверочное слово к слову потонет

1/ She can`t bear to be left alone in the dark, is accustomed as a child to have a light on all night.2/ He does nothing but complains when he is asked to do anything that means to put himself out.3/ He had been studying in London for two years when I come here.4/ By next July she will have been living here for five years.

проверочное слово к слову потонет= тОнет( проверяем вторую о) , а первую О проверить нельзя, надо знать, что это приставка по.