Step 1: Identify the time frame. 'By the time the teacher arrived' sets a reference point in the past (the teacher's arrival). Step 2: Identify the nature of Siti's action. 'For two hours' shows an ongoing duration leading up to that past reference point. Step 3: Match tense to the structure. An action that was ongoing in duration up to a past reference point requires the past perfect continuous: had been + verb-ing. Answer: had been writing