The vector $\overrightarrow{\mathrm{P}}=\mathrm{a}\widehat{\mathrm{i}}+\mathrm{a}\widehat{\mathrm{j}}+3\widehat{\mathrm{k}}\mathrm{and}\overrightarrow{\mathrm{Q}}=\mathrm{a}\widehat{\mathrm{i}}-2\widehat{\mathrm{j}}-\widehat{\mathrm{k}}$ are perpendicular to each other. The positive value of a is

• A

  3

• B

  4

• C

  9

• D

  13