In a controlled scientific experiment, a

A variable is any factor, trait, or condition that can exist in differing amounts or types. Anexperiment usually has three kinds of variables: independent, dependent, andcontrolled. The independent variable is the one that is changed by the scientist.

Alpha particles have positive charge, beta particles have negative charge, and answer 4 just doesn’t make sense. thus, you are left with only one possible answer.

Afraidsugar lol it’s the third one