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A facility would like to verify each individual's identity prior to allowing access to the datacenter. Additionally,the facility would like to ensure that users do not tailgate behind other users. Which of the following would BEST meet these goals?

A. Implement a biometric reader at the datacenter entrance and require passage through a mantrap B. Implement a security guard at the facility entrance and a keypad on the datacenter entrance C. Implement a CCTV camera and a proximity reader on the datacenter entrance D. Implement a biometric reader at the facility entrance and a proximity card at the datacenter entrance

The main difference between a pure virtual function and a virtual function is:

a. The return type. b. The member access specifier. c. That a pure virtual function cannot have an implementation. d. The location in the class.