Determine Whether the Expression Is a Polynomial $9 s ^ { 4 } - 7 s - 7$

Determine whether the expression is a polynomial. If the expression is a polynomial, state whether it is a monomial,
binomial, or trinomial. If the expression is not a polynomial, indicate why not.
- $9 s ^ { 4 } - 7 s - 7$

A) monomial
B) trinomial
C) not polynomial, no variable in last term
D) binomial