Which of the following computer devices are output devices only?

A. Track ball
B. CRT monitor
C. Number pad
D. Plotter
Choose the correct answer from the options given below:

The correct option is B and D only.

To understand why this is the correct choice, let's analyze each of the computer devices listed in the question and classify them as input or output devices:

A. Track ball:
A trackball is a pointing device consisting of a ball held by a socket containing sensors to detect a rotation of the ball. It is used to input spatial data into a computer, similar to a mouse. Therefore, it is an input device only.

B. CRT monitor:
A Cathode-Ray Tube (CRT) monitor is a classic display device that receives video signals from the computer and displays them as visual information on a screen. It only outputs visual data to the user and does not accept user input. Therefore, it is an output device only.

C. Number pad:
A number pad (or numeric keypad) is a section of a keyboard containing numbers and basic mathematical operators. It is used to input numeric data into the computer. Therefore, it is an input device only.

D. Plotter:
A plotter is a vector graphics printing device that interprets commands from a computer to draw line drawings on paper using one or more automated pens. It produces physical hard copies of digital designs. Therefore, it is an output device only.

By evaluating all the options:
- CRT monitor (B) and Plotter (D) are the only devices that function exclusively as output devices.
- Track ball (A) and Number pad (C) function exclusively as input devices.

Thus, the combination containing output devices only is B and D only.