What is the formula to use for this question, "how far does light travel in 2.5s"?

The definition of the speed of light is exactly 299,792,458 meters per second, so to find how far it travels in a time period, multiply the speed of light times the time. Aka c=299,792,458m/s where c is speed of light, m is meters, and s is seconds. So for example to find how far light travels in 5 seconds, multiply by 5.

1) according to the law of conservation of mass ,in all chemical reactions total mass is unchanged

Large asteroid and comet impacts. because they can have a huge impact on the environment, such as a loss of land, and crust on the earth with the big holes they produce. on the other hand, volcanic eruptions can also cause change within the environment by producing land, but im actually not quite sure which one would be a better answer, hopefully you read this and didnt just flat out choose the first sentence i have spoken! your welcome~