a) The appropriate null and alternative hypotheses are:
\[
H_0: \mu = 20 \quad \text{versus} \quad H_1: \mu > 20.
\]
\[
\boxed{H_0: \mu = 20 \quad \text{versus} \quad H_1: \mu > 20}
\]
b) The requirements to perform the test using the \(t\)-distribution are satisfied by:
\[
\boxed{\text{B, C, and D}}
\]