In procedural programming, variables must be declared before they can be used for assignment or any other operation. The steps "Set Diff = x - y," "If y > x, set Diff = y - x," and "Put Diff to output" all presuppose that a memory location named Diff has already been established. Therefore, the declaration of the variable Diff is the logical and necessary first step in the algorithm's sequence.

A. Outputting a result is a terminal step that occurs after all necessary computations are complete.

B. This assignment operation requires the variable Diff to have been declared first.

C. This conditional assignment also cannot be performed until the variable Diff has been declared.

1. Stroustrup, B. (2013). The C++ Programming Language (4th ed.). Addison-Wesley. Section 2.2, "Variables and Basic Types," explains that a variable must be declared to associate a name with a piece of memory before it can be used.

2. Downey, A. B. (2015). Think Python: How to Think Like a Computer Scientist (2nd ed.). O'Reilly Media. Chapter 2, Section 2.1, "Assignment statements," notes that a variable must be created (which in many languages is done via declaration) before a value can be assigned to it.

3. Kernighan, B. W., & Ritchie, D. M. (1988). The C Programming Language (2nd ed.). Prentice Hall. Chapter 1, Section 1.2, "Variables and Arithmetic Expressions," states, "All variables must be declared before use."