2. Controlled experiments are required for performing experiments. To test the toxicity of a chemical on cell viability, which of the following treatments is the most proper control treatment? The chemical is dissolved in 0.1% DMSO and added 1 µL into 3 mL of cells. (A) Add nothing to the cells. (B) Add 1 µl water to the cells. (C) Add 1 µL of 0.1% DMSO to the cells. (D) Add 1 µL of 1% DMSO to the cells.