Question

If ${}^{21}{C}_1+5\cdot {}^{21}{C}_5+9\cdot {}^{21}{C}_9+\dots .+17\cdot {}^{21}{C}_{17}+21\cdot {}^{21}{C}_{21}=k$. Then number of prime factors of $k$ is

• A. 1
• B. 2
• C. 4
• D. more than 4

Answer 1

Answer: (C)

Correct answer is (c) 4