The results of an investigation are usually more reliable if A.
it is done outside.
B.
it is done only once.
C.
it is done using a ruler.
D.
it is done more than once.

A test will be reliable when it gives the same repeated result under the same conditions.

D. it is done more than once

Explanation:

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.