In a memo to all those involved in tax preparation, the IRS indicated that the mean amount of refund was $1600 with a standard deviation of $850. Assume the distribution of the amounts returned follows the normal distribution. What percent of the refunds were less than $900? Select one: Oa. 0.2061 Ob. 0.7939 Oc 0.1131 Od. 0.2939 Oe. 0.6808