The magnitude of the force between $+1$ C and $-1$ C charges placed 10 cm apart is —

• a$9 \times 10^{11}$ N (attractive)
• b$9 \times 10^{11}$ N (repulsive)
• c$9 \times 10^{9}$ N (attractive)
• d$9 \times 10^{7}$ N (attractive)