This question's explanation and your answers so far.
昨年1月1日に本番稼働を開始したソフトウェアの保守作業の件数を1月から12月まで月別に集計したところ，図のようなグラフになった。このグラフにおける要因Aに該当する保守作業のうち，最も適切なものはどれか。 図: 縦軸は「ソフトウェア保守作業の件数」，横軸は「1月」～「12月」の積上げ棒グラフ。各月の棒は2区分で，凡例は「要因A以外の件数」（白）と「要因Aで発生した件数」（網掛け）。網掛けの「要因Aで発生した件数」は稼働直後の1月が最も大きく，月を追うごとに減少し，年の後半にはほとんど見られなくなる。
If it were a one-off task carried out only once in June of last year, the graph should show a peak in June (and immediately after). However, the shaded portion is largest in January and by June there is already only a tiny amount, with no sign whatsoever of an increase in June. The way a one-off event appears does not match the shape in the figure, which is largest right after the start of operation and decreases steadily, so this choice is wrong.
Requests for improvement are raised as users actually use the software more and more, so normally they are few right after operation starts and then either increase over time or occur sporadically throughout the year. That is the opposite of the movement in the figure, where the count is largest in January right after operation starts and disappears in the latter half of the year, so it does not explain factor A.
正解 In the figure, the shaded portion (the count occurring due to factor A) is largest in January, right after production operation begins; it is also large in February and March, becomes smaller month by month from April onward, and hardly appears at all from August. In other words, factor A has the property of being greatest at the point operation starts and then decreasing over time until it disappears. Software initial defects (defects that testing before release failed to remove and that therefore remained) show up in a concentrated way right after production operation begins, when many users actually start using the software, and the number of occurrences falls as fixes progress. This shape of largest right after the start of operation, then decaying and vanishing, is exactly the typical way initial defects appear, so the maintenance work corresponding to factor A is ウ, work to address the software's initial defects. In addition, the white portion (the count of factors other than A) rises and falls throughout the year with no particular trend, which also supports the conclusion that factor A, concentrated only right after the start of operation, is initial defects.
If it were routine data addition work carried out every April, the count should be concentrated in April (or around it) and form a peak. However, in the figure the shaded portion is larger in January to March than in April, and there is no prominent peak in April. The periodic pattern of a yearly routine task cannot explain the decreasing trend in the figure, so this choice is wrong.