You grow neurons in culture in the presence or absence of nerve growth factor (NGF). You perform this experiment using either wild-type neurons or neurons lacking caspase-3, an effector protein required for programmed cell death by apoptosis in these cells. After a day, you examine cell survival in the culture using a microscope. The survival scores (percentage of cells surviving the treatment) are presented in the following table. Which column (a or b) corresponds to the experiment in the presence of NGF? Which row (1 or 2) corresponds to the experiment using wild-type cells?