Choice Dhoice C is the correct answer. When an equation equals zero, we solve using the same strategy.

1. Start with the equation: $x - 9 = 0$
2. Add 9 to both sides: $x = 0 + 9$
3. Simplify: $x = 9$
4. Verify: $9 - 9 = 0$ ✓

This demonstrates a simple principle: when $x - a = 0$, the solution is always $x = a$. This is because only $a - a = 0$.

💡 Zero Equations: The equation $x - a = 0$ is equivalent to $x = a$. Any number minus itself equals zero.

Choice Dhoice C is incorrect because zero is the result of the equation, not the value of $x$. Don't confuse the right side with the solution.

Choice Dhoice D is incorrect because this is the negative of the correct answer. There's no mathematical justification for $x = -9$ here.

Choice Dhoice A is incorrect because 1 is not relevant to this problem. This might result from confusing different mathematical concepts.