Mark.(be) to Sa Pa twice,the first time .....(be) more than ten years old

mark.(be) to Sa Pa twice,the first time .....(be) more than ten years old
how often...(you/do) household chores? every day
twenty years ago, most people around the world (not know) what the internet was
if we dump all sorts of chemicals into rivers,we ( not be able to) swim in them in the future