A: Can you lend me ___a/ an_____ eraser? B: (✓)
A: Can you lend me an eraser, please? B: Sure. Here you go.
A: Can you lend me ___a/ an___ pencil, please? B: (X)
A: Can you lend me a pencil, please? B: Sorry. I don't have one.
A: Can you lend me ___a/ an___ umbrella, please? B: (✓)
A: Can you lend me an umbrella, please? B: Sure. Here you go.