Operations Research Question:

Classify the following variables as "based on observations" or "based on time":

Examples:

- Time for an electronic component to fail. (based on observations)

- Inventory level of an item. (based on time)

Questions:

(a) Number of orders of an item in an inventory.

(b) Number of defective items in a lot.

(c) Time required to grade exams.

(d) Number of vehicles parked in the parking lot of a car rental agency.