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An Identity on Euler Phi Function

NUMTH-HHYZXB

Let $\phi(n)$ be the Euler-Phi function. Complete the following identity:

$$\sum_{d|n} \phi(d) =$$

A

$\phi(n)$

B

$n$

C

$\frac n{\phi(n)}$

D

$n^2$

E

$n-1$