The dielectric strength of air is . . . . . . .

• A
  $3 \times 10^9 \frac{V}{cm}$
• B
  $3 \times 10^6 \frac{V}{m}$
• C
  $3 \times 10^9 \frac{V}{\mu m}$
• D
  $3 \times 10^9 \frac{V}{m}$