Defining $Y _ { t }$ As Current Output $\bar { Y } _ { t }$

Defining $Y _ { t }$ as current output, $\bar { Y } _ { t }$
As potential output, and $\tilde { Y } _ { t }$
As short-run fluctuations, which of the following equations does the text use to measure the fluctuations component of output?

A) $\tilde { Y } _ { t } = \frac { Y _ { t } + \bar { Y } _ { t } } { \bar { Y } _ { t } }$
B) $\tilde { Y } _ { t } = Y _ { t } - \bar { Y } _ { t }$
C) $\tilde { Y } _ { t } = \frac { Y _ { t } - \bar { Y } _ { t } } { \bar { Y } _ { t } }$
D) $\tilde { Y } _ { t } = \frac { \bar { Y } _ { t } - Y _ { t } } { \bar { Y } _ { t } }$
E) $\tilde { Y } _ { t } = \frac { \bar { Y } _ { t } - Y _ { t } } { Y _ { t } }$