We rarely know at the start of an experiment what extraneous variables could affect our results. The best means of controlling unknown sources of extraneous variables is Group of answer choices

Group of answer choices as it was omitted

a. matching

b. randomizing

c. counterbalancing

d. analysis of co-variance

Randomizing

Explanation:Randomizing, a statistical theory in the design of experiment generally used in randomized controlled trials of experimental research is defined as a method which involves randomly  assigning  experimental units to any treatment group which is based on chance and not choice. Randomization reduces  or  eliminate  bias and controls  extraneous variables (an unnecessary variable which is not the  Independent Variable that can influence the dependent variable) that could affect results giving a more reliable results.

answer; no longer than a reasonable period of time;

answer; ///attempts to change more than once behavior at the same time;