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A single alternative decision structure is a control structure in programming that uses a conditional statement to determine whether a specific block of code should be executed or not. It consists of an 'if' statement with a condition; if the condition is met (evaluated as True), the code block within the structure is executed. If the condition is not met (evaluated as False), the code block is not executed and the program continues with the next line of code after the structure.

Let's consider an example in Python programming language to demonstrate a single alternative decision structure: ```python user_age = int(input("Enter your age: ")) if user_age >= 18: print("You are allowed to vote.") ``` In this example, the flow of the program works as follows: 1. The user is prompted to enter their age, which is stored in the variable 'user_age'. 2. An 'if' statement checks whether the value of 'user_age' is greater than or equal to 18. 3. If the condition is True (user_age >= 18), the program will execute the print statement, "You are allowed to vote.". 4. If the condition is False (user_age < 18), the print statement will not be executed, and the program will move on to the next line of code after the structure (if there is any). In this case, the single alternative decision structure helped in determining whether a user is eligible to vote or not depending on their age.