An $\text{LED}$ emitting at $1\;\mu\text{m}$ with a spectral width of $50\;\text{nm}$ is used in a Michelson interferometer. To obtain a sustained interference, the maximum optical path difference between the two arms of the interferometer is
- $200\;\mu\text{m}$
- $20\;\mu\text{m}$
- $1\;\mu\text{m}$
- $50\;\text{nm}$