Which of the following is electron-deficient?

• A

  ${\left({\mathrm{BH}}_3\right)}_2$

• B

  ${\mathrm{PH}}_3$

• C

  ${\left({\mathrm{CH}}_3\right)}_2$

• D

  ${\left({\mathrm{SiH}}_3\right)}_2$