Which of the following species is/are having only one corner oxygen shared per tetrahedron?

 Are the species  having only one corner oxygen shared per tetrahedron.

$\begin{array}{r}{\mathrm{Si}}_2{\mathrm{O}}_7^{2-}\end{array}$, $\begin{array}{r}{\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}\end{array}$, ${\mathrm{S}}_2{\mathrm{O}}_7^{2-}$ and ${\mathrm{P}}_2{\mathrm{O}}_7^4$.

Thus, the correct option are a,b,c and d.