Explain the need to postulate the existence of dark matter in the universe.

Ineed with these last two questions: 1) what are two types of cold air masses that influence the weather in north america? name them and describe their humidity. 2) describe the motion of a cold air mass and a warm air mass and the weather you would expect at a warm front and an occluded front.

Which is not a characteristic of a heat pump? a. it is used in air conditioners. b. it uses a refrigerant. c. it requires no work. d. it reverses the normal flow of thermal energy.

What is the freezing point of radiator fluid that is 50% antifreeze by mass? kf for water is 1.86 ∘c/m.