Complete-the-passage. The connector is the word "because," so the blank must supply the reason that makes the puzzling result logical. The puzzle is that each self-checkout transaction is faster, yet the average time a customer spends waiting did not fall. A genuine explanation must show why faster individual transactions failed to translate into shorter overall waits.
(C) does exactly this. Overall waiting time depends on total throughput, that is, how many customers the whole front-end can process per hour, not just on how fast one transaction is. If the store removed several staffed lanes to make room for the machines, the total number of checkout stations stayed roughly the same, so faster individual transactions were offset by the loss of capacity that the removed lanes had provided. Plugged into "because," (C) makes the flat wait time the expected result. It is the only completion the argument logically requires.
(A) simply repeats the speed fact the stem already states. It is the answer a reader grabs by anchoring on the most prominent fact in the passage, but a restated premise explains nothing; if anything it makes the unchanged waits more surprising, not less.
(B) is drawn from the most recently mentioned idea, time spent waiting. What shoppers do while standing in line does not affect how quickly the line clears, so it leaves the puzzle untouched.
(D) takes a plausible-sounding behavioral trend and runs past the argument's needs. Buying fewer items would make self-checkout transactions even quicker, which would tend to shorten waits, the wrong direction for explaining why they stayed flat.
(E) is the strongest distractor because it names a real slowdown mechanism in assertive language. But "occasionally" jamming cannot, on its own, fully cancel a consistent per-transaction speed advantage; the argument never mentions reliability, and a contingent, partial drag does not carry the logical weight that "because" demands. Only a structural fact about total capacity, as in (C), supplies the required reason.
The correct answer is (C).