A linear function $f$ is known to satisfy $f\left(1\right)=3$ and $f\left(3\right)=7$. Find the constants $a$ and $b$ so that $f\left(x\right)=ax+b$.

• A. $a=4$, $b=-1$
• B. $a=2$, $b=2$
• C. No unique constants exist
• D. $a=2$, $b=1$