What is the relationship between a problem statement and the hypothesis?

. i know what each of them are.

An hypothesis is the proposed explanation that is made on the basis of limited evidence and it serves as the starting point for further investigation while a problem statement is a clear, concise description of the problems which needs to be addressed. 
Problem statement is used to concentrate the focus of the researcher at the beginning, to keep him on track during the operation  and at the end it is used to evaluate if the process delivered an outcome that solve the problems.
Hypothesis is used to define the relationship between two variables at the end of the research the scientist has to decide when its starting hypothesis is valid or not.

C. definite shape, definite volume. that is a solid.

The population of unionid mussels decreased after zebra mussels were introduced. zebra mussels competed with the unionid mussels for food. the zebra mussels were able to displace the unionid mussels, allowing the zebra mussel population to increase  over the years. the unionid mussel population decreased because  of lack of food.