riddle for u guys since im bored / I have cities, but no houses. I have mountains, but no trees. I have water, but no fish. What am I? / brainliest if right

Two technicians are discussing how to test a car's exhaust by holding a small piece of paper within an inch of the tailpipe mouth. technician a says that if the paper is vibrating due to a "puffing" output from the tailpipe, this signals a smoothly running engine. technician b says that if the paper is being drawn toward the tailpipe, this could indicate burned exhaust valves. who is correct? a. technician b b. both technicians a and b c. technician a d. neither technicians a or b

Earth's atmosphere is unique among the planets and is able to support life because it contains a large amount of a) oxygen b) helium c) methane d) carbon dioxide

Apopulation of bolts has a mean thickness of 20 millimeters, with a population standard deviation of .01 millimeters. give, in millimeters, a minimum and a maximum thickness that includes 68% of the population of bolts.