The University College is interested in the average number of hours per week that freshmen spend going to parties. They took a random sample of $250$ freshmen and calculated a mean of $7\mathrm{.}5$ hours a week going to parties and a $95\%$ confidence interval for the population mean to be $2\mathrm{.}5$ to $12\mathrm{.}5$ hours a week. Based on this information, what was their standard error of the mean?

$1\mathrm{.}25$

$5$

$1\mathrm{.}75$

$2\mathrm{.}5$