Which statement must be true?
a. a statement cannot become a theorem until it is proven.
b. a statement cannot become a proof until it is a theorem
c. a statement cannot become a theorem until it becomes a postulate
d. a statement cannot become a postulate until it is proven

1. according to the internal revenue service, the mean tax refund for the year 2007 was $2,708. assume the standard deviation is $650 and that the amounts refunded follow a normal probability distribution. a. what percent of the refunds are more than $3,000? b. what percent of the refunds are more than $3,000 but less than $4,000? c. what percent of the refunds are less than $2,000?