Because cold air tends to more dense, and it's therefore heavier, and it sinks.
Hot air though is hot with no density so it's light and rises.
I agree with the other comment
Answer:
The momentum would be doubled
Explanation:
The magnitude of the momentum of the freight train is given by:

where
m is the mass of the train
v is its speed
In this problem, we have that the speed of the train is unchanged, while the mass of the train is doubled:

therefore, the new momentum is

so, the momentum has also doubled.
Answer:
a). Single replacement.
Explanation:
Because one element replaces another element in a compound
isotopes are the same element, but have different numbers of neutrons (but still have the same number of electrons and protons), hence have a different mass number.