Which of the following reflects a weak internal control system? Please explain with full explanation. a. All employees are well supervised. b. A single employee is responsible for comparing a...

Which of the following reflects a weak internal control system? Please explain with full explanation.

a. All employees are well supervised.

b. A single employee is responsible for comparing a receiving report to an invoice.

c. All employees must take their vacations.

d. A single employee is responsible for collecting and recording cash.