Final answer:
The ampersand (&) and asterisk (*) are used to indicate whether a variable is being passed by value or by reference in programming languages such as C or C++.
Step-by-step explanation:
The ampersand (&) is used to indicate that a variable is being passed by reference in programming languages such as C or C++. When a variable is passed by reference, the actual memory address of the variable is passed to the function, allowing the function to modify the original value.
On the other hand, the asterisk (*) is used to indicate a pointer variable in C or C++. A pointer variable stores the memory address of another variable. When a pointer variable is passed to a function, the function can access and modify the value at that memory address.
Neither the dollar sign ($) nor the exclamation mark (!) have any direct association with indicating whether a variable is being passed by value or by reference in programming languages.