(+) I have never cut the tree in the park.(-) He hasn't cleaned the parks and forest yet.(+) She has already stayed on the path during the nature walk.(-) They haven't polluted the air and water in this area before.(?) Have you ever thrown rubbish away in the park?(+) I have just cut a small branch from a tree in the forest.(-) He hasn't cleaned the parks and forest for a long time.(+) She has already thrown the litter in the trash can.(-) They haven't stayed on the path as instructed.(?) Have you ever polluted the air and water by burning waste?(+) I have never cut down any trees in my backyard.(-) He hasn't thrown away the rubbish properly in the designated bins yet.(+) She has just started cleaning the parks and forest regularly.(-) They haven't stayed on the path in order to protect the fragile ecosystem.(?) Have you ever polluted the air and water knowingly?