An object cannot remain at rest unless which of the following holds? the net force acting on it is zero. the net force acting on it is constant. there are no forces at all acting on it. there is only one force acting on it.

An object cannot remain at rest unless the net force acting on it is zero.  It also can't move with constant, uniform motion (in a straight line at constant speed).

If the net force on it is anything other than zero, then the object must accelerate.  (Either its speed, or its direction, or both, must be changing.)  

the best answer for these questions would be:

1.            c. is smaller

2.            d) concrete operational

3.            b) interaction with the environment – the children interact with their environment can them learn more.

4.            a) parents who reason openly with children can limit a child’s cognitive ability.