Select the equivalent expression. \left(\dfrac{b^{7}}{4^{5}}\right)^{ -3}=?( 4 5 b 7 ​ ) −3 =?Left parenthesis, start fraction, b, start superscript, 7, end superscript, divided by, 4, start superscript, 5, end superscript, end fraction, right parenthesis, start superscript, minus, 3, end superscript, equals, question mark Choose 1 Choose 1 (Choice A) A \dfrac{b^{21}}{4^{15}} 4 15 b 21 ​ start fraction, b, start superscript, 21, end superscript, divided by, 4, start superscript, 15, end superscript, end fraction (Choice B) B \dfrac{4^{15}}{b^{21}} b 21 4 15 ​ start fraction, 4, start superscript, 15, end superscript, divided by, b, start superscript, 21, end superscript, end fraction (Choice C) C {b^{-21}}\cdot{4^{-15}}b −21 ⋅4 −15 b, start superscript, minus, 21, end superscript, dot, 4, start superscript, minus, 15, end superscript