NASA has been planning a manned trip to Mars since the 1990's. Which of the following is a reason why space travel is so difficult?

A- large distance between the planets
B- the cold temperatures of space
C- inability to find a landing zone
D- unpredictable movement of the planets

please hurry!!

Butane gas reacts with oxygen gas to give carbon dioxide gas and water vapor (gas). if you mix butane and oxygen in the correct stoichiometric ratio, and if the total pressure of the mixture is 390 mmhg, what is the pressure (in mmhg) of water vapor after the reaction is completed (temperature and volume do not change).

Is it safe to take 450mg of diphenhydramine hydrochloride?

In chemistry, the type of an atom (what element it is) is determined by: a) the number of protons it contains in its nucleus.b) the number of neutrons it contains in its nucleus.c) the number of protons it has in a cloud around the nucleus.d) the number of neutrons it has in a cloud around the nucleus.e) the number of electrons it exchanges with its neighbors.