In $but-2-ene$,which one of the following statements is true?

• A
  $C_1-C_2$ bond is a $sp^3-sp^3$ $\sigma$-bond
• B
  $C_2-C_3$ bond is a $sp^3-sp^2$ $\sigma$-bond
• C
  $C_1-C_2$ bond is a $sp^3-sp^2$ $\sigma$-bond
• D
  $C_1-C_2$ bond is a $sp^2-sp^2$ $\sigma$-bond