It becomes, then, a question of motive.

1 seconds sound clip from the Phantom of the Opera (1943) movie soundboard.

You can hear this line at 00:53:25.020 in the Blu-ray version of the movie.

Quote context

[...]

- I assure you, monsieur, the property man swears that there was no opportunity for any human being to tamper with the drinks.

- Monsieur Inspector, what are you waiting for? I demand the arrest of Anatole Garron. You know he did it.

- I know nothing of the sort, madame. I am a police officer, not a psychic. It is my duty to collect evidence, without prejudice.

- Well, haven't you evidence enough? Everyone knows...

- Madame! Will you be seated, please?

- It is true, Monsieur Garron, that you had the opportunity of placing the drug in Madame Biancarolli's glass.

- Certainly, Monsieur Inspector. We all did.

- It becomes, then, a question of motive.

- The motive is very simple, monsieur. He wanted to get me out of the way so he could make room for that little...

- Are you referring to Mademoiselle DuBois?

- I am.

- You heard, Monsieur Garron.

- Madame is in good voice, and most explicit.

- Have you anything to say, monsieur?

[...]