Step 1: Identify the tense and structure of the active sentence. Active: The volunteers (subject) cleaned (simple past) the beach (object) last Saturday. Step 2: Apply the passive voice formula for simple past. Passive formula: Object + was/were + past participle + by + agent + time expression. Object = the beach (singular) → use "was" Past participle of clean = cleaned Step 3: Construct the passive sentence. The beach was cleaned by the volunteers last Saturday. Step 4: Verify tense consistency. The time marker "last Saturday" confirms simple past — "was cleaned" is correct. Answer: The beach was cleaned by the volunteers last Saturday.