Which of the following conditions are valid at the plait point?

1. Density difference between the extract and raffinate phases is zero
2. Interfacial tension  between the extract and raffinate phases is zero
3. Composition difference between the extract and raffinate phases is zero
1. $P$ and $Q$ only
2. $Q$ and $R$ only
3. $P$ and $R$ only
4. $P$, $Q$ and $R$