Which of the following conditions are valid at the plait point?
- Density difference between the extract and raffinate phases is zero
- Interfacial tension between the extract and raffinate phases is zero
- Composition difference between the extract and raffinate phases is zero
- $P$ and $Q$ only
- $Q$ and $R$ only
- $P$ and $R$ only
- $P$, $Q$ and $R$
