Answer:
First Condition of Equilibrium is that it must be experiencing no acceleration or the external forces acting on the body should be zero.
What did the protoplanets become?
a. nebulae
b. planets
c. solar nebulae
d. planetesimals
The protoplanets
become nebulae. The answer is letter A. The
rest of the choices do not answer the question above.

When an opaque obstacle is placed between a source of light and a screen, a shadow of the obstacle is formed on the screen. The kind of shadow depends on the size of the source of light. In other words, the earth casts its shadow on the moon. The solar eclipse occurs when the moon comes between the sun and the earth.
Answer:
Sound energy is produced when an object vibrates. The sound vibrations cause waves of pressure that travel through a medium, such as air, water, wood or metal. Sound energy is a form of mechanical energy.
Explanation: