I can't understand this subject so well.

A

Explanation:

There are some principles on earth you should know, it is gravitational force. You have heard whatever goes up must come down. This is a force the earth exacts on any object to keep on the earth surface. It is a constant value on the earth and is 9.8m/s2. Now mass is a property of an object that gives it it's form or rigidity. You have heard something is massive means it's heavy. So this properties mass and gravitational pull are independent. As you move higher you would see a relationship when we which to create something that can fly in that case we would try to counter this gravitational pull.