A pharmacist receives a medication order to prepare different concentrations of penicillin $\mathrm{G}$ potassium to be added to $1000 \mathrm{~mL}$ of D5W, using stock vials of $1,000,000$ units of penicillin G sodium. The instructions on the vials of penicillin G potassium stated, "Add 4.6 milliliters of sterilized water for a concentration of $2.5 \times 10^5$ units $/ \mathrm{mL}$." How many milliliters of the reconstituted solution must be added to the D5W to provide the following units of penicillin G sodium in the final preparation?
a. $2 \times 10^5$ units
b. $4 \times 10^5$ units
c. $5 \times 10^5$ units
d. $6 \times 10^5$ units
e. $2 \times 10^6$ units
f. $1 \times 10^6$ units