Occur when two meet.
a) air masses, fronts
b) fronts, air masses
c) cyclones, fronts
d) fronts, clouds

B) Fronts, air masses

When the air masses meet fronts are formed. These fronts reflects the weather condition of a region.  

Cold front forms when a cold air mass slides under a warm air mass, resulting in the warm air mass to rise above. This is responsible for cold weather conditions.  

Warm front forms when a moist warm air mass slides over the cold air mass. This is responsible for gentle rain and snow.

Stationary front is forms when warm and cold air neither meet nor able to displace each other. These air masses remain in the stationary condition. This is responsible for cloud formation and precipitation.  

Occluded front is formed when a warm air mass gets caught between two cold air masses. This is responsible for strong wind and heavy precipitation.  

i'd say that the moon rock is about 2.2 billion years old.

blue stars have the highest surface temp

stellar distances are usually expressed in light-years

the measure of a star's brightness is called magnitude

hope this !