Which choice correctly defines an arithmetic sequence?

Given the sequence $3$, $7$, $11$, $15$, what is the common difference ($d$)?

Write the explicit $n$^th-term formula, $a_n$, for an arithmetic sequence with $a_1=5$ and $d=4$.

For the arithmetic sequence with $a_1=2$ and $d=4$, compute $a_{20}$.

A researcher increases the dosage of a drug to a test group by a fixed 2 mg each day. The first day the dose is 5 mg. On what day n will the dosage first exceed 50 mg?