C. element only one substance
Answer:
Associations Clusters
Explanation:
There is three basic type of clusters, globular clusters, open clusters, associations clusters. Associations cluster consist of younger stars that is younger than globular and open clusters stars.
Answer:
Two equal and opposite parallel forces not acting along the same line, form a couple. A couple is always needed to produce the rotation.
For example, turning a key in a lock and turning a steering wheel.
Answer:
Maximum height reached by the rocket is

total time of the motion of rocket is given as

Explanation:
Initial speed of the rocket is given as

acceleration of the rocket is given as

engine stops at height h = 150 m
so the final speed of the rocket at this height is given as



so maximum height reached by the rocket is given as the height where its final speed becomes zero
so we will have



Now the total time of the motion of rocket is given as
1) time to reach the height of 150 m



2) time to reach ground from this height



so total time of the motion of rocket is given as

Answer:
B. 
Explanation:
Assuming we are dealing with a perfect gas, we should use the perfect gas equation:

With T the temperature, V the volume, P the pressure, R the perfect gas constant and n the number of mol, we are going to use the subscripts i for the initial state when the gas has 20 cubic inches of volume and absolute pressure of 5 psi, and final state when the gas reaches 10 psi, so we have two equations:
(1)
(2)
Assuming the temperature and the number of moles remain constant (number of moles remain constant if we don't have a leak of gas) we should equate equations (1) and (2) because
,
and R is an universal constant:
, solving for 

