Which completes the syllogism?

if the time is between 9: 00 am and 5: 00 pm, then the post office is open.
if the post office is open, then people may buy stamps or mail packages.
therefore, if …

a. people may buy stamps or mail packages, then they can also send express mail too.

b. the post office is open, then it is friday.

c. the time is between 9: 00 am and 5: 00 pm, then many people are likely at work and unable buy stamps or mail packages.

d. the time is between 9: 00 am and 5: 00 pm, then people may buy stamps or mail packages.