Answer:
use two marbles that are having the same mass.
Step-by-step explanation:
An experiment can be defined as an investigation which typically involves the process of manipulating an independent variable (the cause) in order to be able to determine or measure the dependent variable (the effect).
This ultimately implies that, an experiment can be used by scientists to show or demonstrate how a condition causes or gives rise to another i.e cause and effect, influence, behavior, etc in a sample.
Compare and contrast is used for the comparison of two or more things to show similarities and differences i.e how they are alike and different.
In this scenario, Emma compares the time it takes for a large marble and a small marble to roll down a slope.
In order to make his findings reliable, Emma should use two marbles that are having the same mass i.e two large marbles with the same mass and two small marbles with the same mass.
Mass can be defined as a measure of the amount of matter an object or a body comprises of. The standard unit of measurement of the mass of an object or a body is kilograms. Irrespective of the location of an object or a body at a given moment in time, the mass (amount of matter that they're made up of) is constant.