$A$ conductor of length $1.5 \ m$ and area of cross-section $3 \times 10^{-5} \ m^2$ has an electrical resistance of $15 \ \Omega$. The current density in the conductor for an electric field of $21 \ Vm^{-1}$ is
- A$0.7 \times 10^6 \ Am^{-2}$
- B$0.7 \times 10^{-6} \ Am^{-2}$
- C$0.7 \times 10^{-5} \ Am^{-2}$
- D$0.7 \times 10^5 \ Am^{-2}$
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