What Concentration of Ca²⁺ Ion Is Required to Trigger the Release

What concentration of Ca²⁺ ion is required to trigger the release of acetylcholine?

A) 1 × 10−⁶ M
B) 1 × 10−⁷ M
C) 1 × 10−⁸ M
D) 1 × 10−⁹ M