In C++, the & symbol after the type in the parameter list does what?

		The & symbol alerts the compiler that the parameter is call by reference.
		The & symbol alerts the compiler that the parameter is call by value.
		The & symbol alerts the compiler that the parameter is a placeholder.
		The & symbol alerts the compiler to swap the actual and formal parameter.

Given: f(n) = nf(n-1) for n>1

f(1)=1

What is f(5)?

		120
		24
		15
		121