The correct answer is $\boxed{\text{A. None follows}}$.
The first statement, “Some mountains are hillocks,” means that there exists at least one mountain that is a hillock. The second statement, “Some mountains are rivers,” means that there exists at least one mountain that is a river. The third statement, “Some mountains are valleys,” means that there exists at least one mountain that is a valley.
The first conclusion, “All mountains are either hillocks or rivers or valleys,” is not necessarily true. For example, there could be a mountain that is not a hillock, not a river, and not a valley.
The second conclusion, “No valley is river,” is also not necessarily true. For example, there could be a valley that is also a river.
The third conclusion, “Some river are valleys,” is also not necessarily true. For example, there could be a river that is not a valley.
Therefore, none of the conclusions follow from the statements.