How many 10-digit numbers can be written by using the digits 1 and 2 ?

• A

  ${ }^{10}{\mathrm{C}}_1{+}^9{\mathrm{C}}_2$

• B

  $2^{10}$

• C

  ${ }^{10}{\mathrm{C}}_2$

• D

  10!