As Shown in the Figure, the Charge $Q$ Is Midway Between Two Other Charges

As shown in the figure, the charge $Q$ is midway between two other charges. If $Q = - 7.5 \mathrm { nC }$ , what must be the charge $q 1$ so that charge $q 2$ remains stationary as $Q$ and $q 1$ are held in place?

A) $15 \mathrm { nC }$
B) $60 \mathrm { nC }$
C) $7.5 \mathrm { nC }$
D) $30 \mathrm { nC }$