What is the C$$t value for $99\mathrm{.}9\%(3\mathrm{.}0$ log$)$ inactivation of Giardia cysts at a free chlorine residual of $1\mathrm{.}0$mg$/$L$,$ temperature of $0\mathrm{.}5,$ and pH $7\mathrm{.}0?$ How does a pH decrease to $6$ affect the C$$t$?$