What is the $100\text{th}$ number in the [arithmetic sequence](https://artofproblemsolving.com/wiki/index.php/Arithmetic_sequence): $1,5,9,13,17,21,25,...$?
$\text{(A)}\ 397 \qquad \text{(B)}\ 399 \qquad \text{(C)}\ 401 \qquad \text{(D)}\ 403 \qquad \text{(E)}\ 405$
To get from the $1^\text{st}$ term of an arithmetic sequence to the $100^\text{th}$ term, we must add the common [difference](https://artofproblemsolving.com/wiki/index.php/Difference) $99$ times.  The first term is $1$ and the common difference is $5-1=9-5=13-9=\cdots = 4$, so the $100^\text{th}$ term is \[1+4(99)=397 \rightarrow \boxed{\text{A}}\]