Step 1: Cavity field.
Solid conductor body: E = 0 inside metal. Cavity with charge Q: \( E_A \ne 0 \) inside cavity.
Step 2: Outside field.
Conductor appears as point charge Q from outside. B and C at same distance from centre: \( E_B = E_C \).
Step 3: Final.
\[ \boxed{E_A \ne 0,\; E_B = E_C} \]