A Random Sample of Size N Has Been Selected from a Normally

A random sample of size n has been selected from a normally distributed population whose standard deviation is . In hypothesis testing for the population mean, the t-test should be used instead of the z-test if:

A) n > 30 and σ is unknown
B) n < 30 and σ is unknown
C) n < 30 and σ is known
D) n < 30 and β is known
E) n > 30 and σ is unknown and n < 30 and σ is known