Theory of complexity: show that APPROX-SAT is in NP

A problem of APPROX-SAT is defined as follows. Given a formula CNF φ with k ≥ 2 clauses
C1 ,. . . , Ck and n variables x1 ,. . . , xn, is there an assignment of the variables such that exactly the clauses k -1 are
true.
(a) Show that APPROX-SAT is in NP.
(b) Show that 3-SAT≤P APROX-SAT.