A language model is given a task where the instruction is 'Summarize the following text:' and the user's input is 'The sun is a star at the center of the Solar System.' If a complete prompt, x, is formally represented as an ordered pair containing the instruction and the user input, which of the following correctly represents x?

The formal expression for a simple prompt, denoted as x, which contains an instruction c and a user input z, is correctly written as x = c + z.

In the formal expression for a simple prompt, represented as x = (c, z), match each variable to its correct description.