数列通项：$a_n = \frac{1}{n} - \frac{1}{n+1}$ A： $a_1 + a_2 + a_3 + a_4$ $= \left(1 - \frac{1}{2}\right) + \left(\frac{1}{2} - \frac{1}{3}\right) + \left(\frac{1}{3} - \frac{1}{4}\right) + \left(\frac{1}{4} - \frac{1}{5}\right)$ $= 1 - \frac{1}{5} = \frac{4}{5}$ A = B 答案：C