(A) The rate of growth or decline in sales of stain-removing agents in city X This tells us if sales of stain-removing agents in City X are going up or down overall. But it doesn’t tell us if the new agent’s smell advantage will make it sell more than the old one. It’s about the market, not about the smell or the two agents. Not very helpful for the argument.
(B) The total number of stain removing agents sold last year in city X This gives us a number of how many agents were sold last year. But it doesn’t say anything about the new agent, the old agent, or how the smell affects sales. It’s just a total, not about why people choose one agent over another. Not helpful here.
(C) The per capita income of people residing in city X This tells us how much money people in City X have on average. Money might affect buying, but the argument is about the smell difference, not price or income. We don’t know if income changes how much people care about the smell. This doesn’t help us check the argument directly.
(D) The amount of time it takes for the smell of the new stain-removing agent to completely go away from the clothes. The argument says the new agent’s smell starts to go away in 2 days, and that’s why it should sell more. But if the smell takes a long time to fully go away—like 9 days—it might still bother people, just like the old agent’s 10 days. If it goes away fast—like in 3 days—it’s a big advantage. Knowing the full time helps us see if the smell difference really makes the new agent better and more likely to sell. This is the most helpful.(E) A comparison of the smell of the new stain removing agent with that of similar agents available in other cities This compares the new agent’s smell to agents in other cities. But the argument is about City X, comparing the new agent to the old one there. Smells in other cities don’t tell us if people in City X will buy the new agent more because of its smell going away faster. Not helpful for this argument.
Answer: D