John works five days each week except when on vacation or during weeks in which national holidays occur. Four days a week he works in an insurance company; on Fridays he works as a blacksmith. Last week there were no holidays, and John was not on vacation. Ttherefore, he must have worked in the insurance company on Monday, Tuesday, Wednesday, and Thursday last week. Which one of the following is an assumption on which the argument depends?
A: Every day last week that John worked, he worked for an entire workday.
B: John does not take vacations in weeks in which national holidays occur.
C: John never takes a vacation of more than one week in length.
D: Last week John worked neither on Saturday nor on Sunday.
Choose A, B, C or D as your solution.
D