Answer: C. Floppy disk drive.
Answer analysis: When discussing this question, we must first clarify the definitions of input devices and output devices. Input devices are devices that allow users to input data or instructions into the computer, while output devices are devices that present the data or results processed by the computer to the user.
Now, let's analyze each option one by one:
A. Keyboard: The keyboard is a typical input device. Users input characters, numbers, instructions, etc. through the keyboard, but the keyboard itself does not have an output function, so it cannot be used as an output device.
B. Monitor: The monitor is an output device used to display the results of computer processing, such as text, images, videos, etc. But it does not have an input function, and users cannot input data or instructions through the monitor.
C. Floppy disk drive: A floppy disk drive is a device that can be used as both an input device and an output device. When the computer needs to read data from a floppy disk, it acts as an input device; when the computer needs to write data to a floppy disk, it acts as an output device. Therefore, the floppy disk drive meets the requirements of the question.
D. Printer: A printer is an output device used to print documents, pictures, etc. processed by a computer onto paper, but it does not have an input function.
In summary, the floppy disk drive can be used as both an input device and an output device. Therefore, the correct answer is C. floppy disk drive.
It should be noted that with the development of technology, floppy disks and floppy disk drives have gradually faded out of the market, but they are still an important example when understanding the basic concepts of computer hardware and input and output devices.