Which of the following is a pyrosilicate.

• A

  ${\text{Sc}}_{\text{2}}{\text{Si}}_{\text{2}}{\text{O}}_{\text{7}}$

• B

  ${\text{Zn}}_{\text{2}}{\text{SiO}}_{\text{4}}$

• C

  ${\text{Ca}}_{\text{2}}{\text{Si}}_{\text{3}}{\text{O}}_{\text{9}}$

• D

  ${\text{Be}}_{\text{3}}{\text{Al}}_{\text{2}}{\text{Si}}_{\text{6}}{\text{O}}_{\text{18}}$