Which of the following is the most likely sign that a chemical change has happened?
a) liquid rests on a table overnight, and some evaporate.
b) two liquids are mixed at room temperature and a gas forms.
c) salt is mixed into water, forming a solution.
d) a solid is crushed into very small pieces.

The correct option is B) two liquids are mixed at room temperature and a gas forms.

Explanation:

A chemical change can be described as a change which results in the formation of a new substance by the reactants. In the option B, two liquids react  to form a new product which is a gas. Hence, it is an example of chemical change.

Other options,like option D, is not a chemical change as by crushing a solid no new product is formed. Option C is also not correct because as when salt dissolves in water, no new product is made.  

in scientific notation, the value of the  speed of light in a vacuum

  will be written as 2.995 * 10^8. when this value is represented in the form of

  a * 10^b, then, a = 2.995 and b = 8.

the value of the speed of light in a vacuum given above can also be rounded up to approximately 300000000. in this case, the scientific notation will be written as 3 *10^8.