A second harmonic has to be generated in a string of length l stretched between two rigid supports. The point where the string has to be plucked and touched are 

• A

  Plucked at $\frac{\mathrm{l}}{4}$ and touch at $\frac{\mathrm{l}}{2}$

• B

  Plucked at $\frac{\mathrm{l}}{4}$ and touch at $\frac{3\mathrm{l}}{4}$

• C

  Plucked at $\frac{\mathrm{l}}{2}$ and touched at $\frac{\mathrm{l}}{4}$

• D

  Plucked at $\frac{\mathrm{l}}{2}$ and touched at $\frac{3\mathrm{l}}{4}$