Which type of wall would make the best soundproofing for room? a. 1.0 centimeter thick glass b. two layers of 0.5 centimeter thick glass with 0.5 centimeters of air between them c. two layers of 0.5 centimeter thick glass with 0.3 centimeters of vacuum between them d. 1.0 centimeter thick steel

The correct answer to this is two layers of 0.5 centimeter thick glass with 0.5 centimeters of air between them 

I think the answer is B.
Look in your book for a table that talks about a 'coefficient of insulation' or something like that. Once you have the coefficients for air, glass and steel, and run through the combo's and scenarios to see which would result in the least transference of waves( less wave movement that will carry sound).
Hope this helps!:)

The independent variable would be the fertilizer as it is what the dependent variable (the growth of grass) depends on.