$A$ solenoid has a core made of material with relative permeability $400$. The magnetic field produced in the interior of the solenoid is $1.0 \text{ T}$. The magnetic intensity in $SI$ units is $\alpha \times 10^5$. The value of $\alpha$ is . . . . . . . (Free space permeability $\mu_0 = 4\pi \times 10^{-7} \text{ SI units}$.)

• A
  $\frac{25}{\pi}$
• B
  $\frac{1}{16\pi}$
• C
  $\frac{1}{\pi}$
• D
  $\frac{1}{4\pi}$