Overcrowded means that there are too many people in a space. This can be a problem in restaurants, buses, and other places where people are supposed to be able to move around freely.
Industrialize is to turn a country or region into a place where factories and other industrial businesses are common. This often leads to a lot of pollution and other problems.
To conceal something is to keep it hidden from view. You might conceal your anger or your sadness by pretending to be happy or content.
To generalize is to make a generalized statement about a group of people or things. For example, you might generalize about all women by saying that they are always emotional.
To rack your brain is to try to think of something that you haven't thought of before. When you're stuck on a problem, you might rack your brain to see if you can come up with a solution.
Unsolvable means that something cannot be solved. This might be because the problem is too big or because there is no solution that is satisfactory to both sides.
To quack is to make a noise like a duck. When someone quacks, it usually means that they are unhappy or disappointed.