How did the French position threaten the American colonists?

The French threatened not only the British Empire but the American colonists themselves, for in holding the Mississippi Valley, France could limit their westward expansion.In the Peace of Paris, signed in 1763, France relinquished all of Canada, the Great Lakes and the upper Mississippi Valley to the British.

Found this myself:

Which British policy convinced many American colonists to want to form their own country?

A. Great Britain separated the colonial governments into three branches.

B. Great Britain ignored the colonies during the French and Indian War.

C. Great Britain did not allow the colonists to participate in government.

D. Great Britain planned to sell several of the colonies to the French

The correct answer is C.