This question's explanation and your answers so far.
E 社の情報システム部では，新しい販売管理システム（以下，新販売管理システムという）の導入を計画している。情報システム部の新入社員 F さんは，既存の販売管理システムから新販売管理システムへの切替作業について，先輩の指導の下に作業時間の見積りを行うことになった。作業計画の一覧は，次の表に示すとおりである。 表 作業計画の一覧
| 作業番号 | 作業項目 | 作業予定時間（時間） | 先行作業番号 |
|---|---|---|---|
| ① | システムの停止 | 0.5 | なし |
| ② | システムのバックアップ | 2 | ① |
| ③ | 移行データの抽出 | 1 | ① |
| ④ | ソフトウェアのインストール | 3 | ②，③ |
| ⑤ | システムの設定作業 | 2 | ④ |
| ⑥ | 移行データの投入 | 3 | ④ |
| ⑦ | 動作確認表の準備 | 1.5 | ④ |
| ⑧ | 動作確認 | 2 | ⑤，⑥，⑦ |
F さんは，全体の作業時間を求めるために表の内容を整理することにした。F さんは，一連の作業の中で並行して実施可能な作業を抜き出した。並行して実施可能な作業の組合せをすべて挙げているものはどれか。ここで，括弧内は並行して実施可能な作業を示している。
It is true that (②, ③) can be performed in parallel, but (⑤, ⑥, ⑦) also have only ④ as their predecessor and do not depend on one another, so they can be performed in parallel as well. The question asks for the option that lists all the combinations of tasks that can be performed in parallel, so ア, which lists only one of them, is incomplete and therefore wrong.
Treating (④, ⑤, ⑥) as tasks that can be performed in parallel is wrong. The predecessor of both ⑤ and ⑥ is ④, so neither ⑤ nor ⑥ can start until ④ is finished. ④ and ⑤/⑥ are in a predecessor-successor relationship, so they cannot be carried out at the same time.
正解 The correct answer is ウ. Tasks that can be performed in parallel are those that have no predecessor-successor relationship with each other and can be started at the same time once all of their required predecessor tasks are finished. Looking at the predecessor task numbers in the table, ② (Back up the system) and ③ (Extract migration data) both have only ① as their predecessor, and there is no dependency between ② and ③. Therefore, once ① is finished, these two can be started at the same time. In the same way, ⑤ (Configure system settings), ⑥ (Load migration data), and ⑦ (Prepare the operation-check sheet) all have only ④ as their predecessor, and there is no dependency among the three. Therefore, once ④ is finished, the three can proceed at the same time. ① is the first task on its own, ④ is the merging point of ② and ③, and ⑧ is the merging point of ⑤, ⑥, and ⑦, so none of them can run in parallel with another task. The only combinations that can be performed in parallel are (②, ③) and (⑤, ⑥, ⑦), and ウ, which lists both of them without omission, is the correct answer.
It is true that (⑤, ⑥, ⑦) can be performed in parallel, but ② and ③ also have only ① as their predecessor and are independent of each other, so they can be performed in parallel as well. エ does not satisfy the condition of listing all the combinations, so it is wrong.