Change the given sentence into passive voice: Who killed the snake?

The correct answer is B) By whom was the snake killed? The active sentence is in simple past (killed), so the passive must be was + past participle. The object, 'the snake', becomes the subject, and the subject 'who' moves into the by-phrase as 'by whom'.

• A is wrong. It swaps the roles, so it says the snake did the killing to someone, which reverses the meaning. The by-phrase must carry the doer of the action.
• C is wrong. 'Was being killed' is past continuous passive, which changes the tense. It also drops the agent, so the question of who killed the snake is lost.
• D is wrong. 'Is killed' is present simple, which does not match the past tense of the original sentence.

The core concept is voice transformation: keep the tense, move the object to the subject position, use the right auxiliary, and retain the meaning. In a question, 'who' becomes 'by whom' inside the passive. Precision of this kind carries over to Mains answer writing, where a change in tense or agent can alter the sense of a statement.

Trap: Candidates pick A because 'who' appears first and looks like the new subject. The question asks for the doer, so 'by whom' must be in the by-phrase.