An insulating sphere of radius $4.0\mathrm{cm}$ is in the center of a conducting spherical shell with inner radius of $5.0\mathrm{cm}$ and outer radius of $7.0\mathrm{cm}$. The insulating sphere has a charge of $-34\mathrm{nC}$ and the conducting spherical shell has a net charge of $42\mathrm{nC}$. What is the total charge on the inner surface of the conducting spherical shell?

• A. $8.0\mathrm{nC}$
• B. $23\mathrm{nC}$
• C. $38\mathrm{nC}$
• D. $42\mathrm{nC}$
• E. $75\mathrm{nC}$
• F. $34\mathrm{nC}$