A PHP variable name a. can contain special characters b. can start with a letter, a digit, or two underscores c. can be a PHP reserved word d. is case-sensitive

D) is case sensitive

Explanation:

All the other given options (A-C) are false about naming variables in PHP programming language. As a matter of fact PHP like other major programming languages have strict rules for naming variables to used in a program. Summarily for a variable to be valid.

the answer to your question would be something called hertz. the unit of measurement called a hertz, which is one cycle per second, is used to measure clock speed. within one cycle, your computer processes, refreshes, or updates itself to the changes that it is going through. in the case of computer clock speed, one hertz equals one tick per second. the clock speed of computers is usually measured in megahertz (mhz) or gigahertz (ghz)