Which character is used as a substitution variable in APEX?

If you want your presentation to contain two or more themes, use multiple ________

Fill in the blank(s) with correct word

Access control at many businesses and organizations is provided by ____, which allow only authorized individuals to enter an office building, punch in or out of work, or access the company network via an access card or a fingerprint or hand scan.

A. bar codes B. virtual reality glasses C. collaboration systems D. authentication systems