In July 1994 the comet shoemaker-levy 9 collided with the planet Jupiter, as scientists had predicted. To be able to accurately predict when a comet will collide with a planet, it is essentially understand -

the gravitational attraction that exists between all bodies

Explanation:

Comet shoemaker-levy 9 was the comet that broke away and collided with the largest planet on our solar system, the Jupiter. The comet fall apart from the asteroids in the year 1992 and flew and hit Jupiter in the year 1994. This was predicted by the scientist.

Similarly when a comet can hit a planet can be accurately predicted by the scientist or the researcher by understanding and studying the gravitational forces that exists between the bodies in the solar system. It is very important to understand the gravitational attractions which exists between all the bodies in the universe in order to calculate or predict when can a comet collide with a planet.

- lead + phosphoric acid = hydrogen + lead(ii) phosphate

the answer is c. nacl