21. Given the following acids and Ka values: HOAc HCN HF HClO₄ 1 x 10⁷ 1.76 x 10⁴ 4.93 x 10⁻¹⁰ 3.53 x 10⁻⁸ Which is correct in term of base strength? (A) CN⁻ &lt; F⁻ &lt; OAc⁻ &lt; ClO₄⁻ (B) CN⁻ &lt; OAc⁻ &lt; F⁻ &lt; ClO₄⁻ (C) ClO4- &lt; OAc- &lt; CN- &lt; F- (D) ClO4- &lt; F- &lt; OAc- &lt; CN-