Rain and storms are caused by
a. low pressure systems
b. high pressure systems
c. extremely low pressure systems

(A) Low Pressure System

Explanation:

Low and High pressure systems have a feel on the weather at a particular area. Consequently, areas with low pressure systems are characterised with frequent rainfall and storms. This is how it works:

At low pressure, the air becomes lighter and less dense making it easy to ascend into the atmosphere. Now because of altitude, it becomes cooled off and then condenses. This condensation results into clouds after mixture with warm and moist air. Somehow due to that chemistry, the cloud formed releases itself and drops as rain with accompanied storms

question

which are human systems?

the amazon rain forest

the appalachian trail

the george washington bridge

the himalayas

the hoover dam

the north american free trade agreement