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A single alternative decision structure is a decision-making structure in programming that executes a specific block of code on the basis of whether a given condition is true or not. In other words, it allows a program to perform an action when a specified condition is met and does nothing if the condition is not satisfied. This type of structure is commonly represented using "if" statements.

A single alternative decision structure can be implemented in various programming languages. Below are examples for Python, Java, and JavaScript. ## Python Example: In Python, the syntax for a single alternative decision structure is as follows: ```python if condition: # code to execute if condition is true ``` For instance: ```python age = 16 if age >= 18: print("You are eligible to vote") ``` In this example, if the variable `age` is greater than or equal to 18, the code will print "You are eligible to vote". If the `age` is less than 18, nothing will happen. ## Java Example: In Java, the syntax for a single alternative decision structure is as follows: ```java if (condition) { // code to execute if condition is true } ``` For instance: ```java int age = 16; if (age >= 18) { System.out.println("You are eligible to vote"); } ``` In this example, if the variable `age` is greater than or equal to 18, the code will print "You are eligible to vote". If the `age` is less than 18, nothing will happen. ## JavaScript Example: In JavaScript, the syntax for a single alternative decision structure is as follows: ```javascript if (condition) { // code to execute if condition is true } ``` For instance: ```javascript let age = 16; if (age >= 18) { console.log("You are eligible to vote"); } ``` In this example, if the variable `age` is greater than or equal to 18, the code will log "You are eligible to vote" in the console. If the `age` is less than 18, nothing will happen.