Assuming S$1$ and S$2$ as interfering sources. Let S$1$ be ahead of the phase by $90$o relative to S$2.$ If an observation point P is such that PS$1-$ PS$2=1\mathrm{.}5\backslash$lambda $,$ The phase difference between the light beam from S$1$ and S$2$ reaching P is a$.3\backslash$pi b$.5\backslash$pi $2$ c$.7\backslash$pi $2$ d$.7\backslash$pi