OPT:
(A). variation.
(B). inertia.
(C). speed.
(D). gravity.

Answer the following question given this paragraph: Every moment of every day is a field trip to gravity. Gravity is everywhere! You have a gravitational attraction to your dog. You have one to your pencil. You even have one to your school principal! These gravitational attractions are very small compared with the most important one you have. This is your gravitational attraction to Earth. It's what keeps you from floating off into space. Gravity holds our planet together. Gravity keeps Earth orbiting the Sun. We wouldn't be here without gravity. Q: What keeps the earth orbiting the sun? A:
Answer:
(D).