This is from Computability and Complexity, Please solve with all the steps

Consider an instance of the Satisfiability Problem, specified by clauses Ci, , Ck over a set of Boolean variables , , xn-we say that the instanceis monotone if each term in each clause consists of a nonnegated variable; that is, each term is equal to x, for some , rather than Monotone instances of Satisfiability are very easy to solve: They are always sausiable, by settirng each variable equal to I For example, suppose we have the three clauses This is monotone, and indeed the assignment that sets all three variables to 1 satisfies all the clauses. But we can observe that this is not the only satisfying assignment; we could also have set and to 1, and x3 to 0. Indeed, for any monotone instance, it is natural to ask how few variables we need to set to 1 in order to satisfy it. Given a monotone instance of Satisfiability, together with a number k, the problem of Monotone Satisfiability with Few True Variables asks: Is there a satisfying assignment for the instance in which at most k variables are set to 1? Prove this problem is NP-complete