Newton concluded that some force must act on the Moon because ____.

Newton concluded that some force must act on the Moon because ____.
 
  a. a force is needed to pull the Moon away from straight-line motion
  b. a force is needed to pull the Moon outward
 c. a force is needed to keep the Moon in motion
 d. the Moon has at a constant velocity
 e. the Moon has a constant acceleration

Question 2

Population I stars are younger than Population II stars.
 
  a. True
  b. False
  Indicate whether the statement is true or false

Question 3

Mutual gravitation is ____.
 
  a. a type of force that acts on an object moving at constant speed
  b. the acceleration of an object in the absence of a force
 c. a property of all matter in the universe
 d. the limit of gravitational force from one object to another
 e. a type of force that repels surrounding particles