Explanation:
Active voice is when someone does something while passive voice is when something is being done by someone. Even though these sound different, the same thing is happening in the story; it's just written differently.
Active: "Johnny bit into an apple."
Passive: "The apple was bit into by Johnny."
These are the same action being performed but written differently, see? With active voice, the main subject is at the beginning. With passive the main subject is at the end.
"Let the assembly began" is passive. Let's switch that around to active. We know that the assembly is beginning.
"The assembly is being let to begin!"
There's the answer.
