$A$ mass $M$ is suspended from a light spring. An additional mass $M_1$ added extends the spring further by a distance $x$. Now the combined mass will oscillate on the spring with period $T=$

• A
  $2 \pi \left[ \frac{M_1 g}{x(M+M_1)} \right]^{1/2}$
• B
  $2 \pi \left[ \frac{(M+M_1) x}{M_1 g} \right]^{1/2}$
• C
  $\frac{\pi}{2} \left[ \frac{M_1 g}{x(M+M_1)} \right]^{1/2}$
• D
  $2 \pi \left[ \frac{M x}{M_1 g} \right]^{1/2}$