A white powder consists of sodium chloride and sodium iodide. It is dissolved in water in a test-tube, then excess aqueous silver nitrate is added. A precipitate, $X$, forms. Next, excess concentrated ammonia is added to the test-tube containing $X$. After shaking the test-tube, a precipitate, $Y$, is seen. Which statement about $X$ or $Y$ is correct?
- A$X$ is a pure white colour.
- B$X$ is pure silver iodide.
- C$Y$ is pure silver chloride.
- D$Y$ is yellow.