$2-$methylpropane on monochlorination under photochemical conditions gives:

• A
  $2-$chloro$-2-$methylpropane as major product
• B
  $1:1$ mixture of $1-$chloro$-2-$methylpropane and $2-$chloro$-2-$methylpropane
• C
  $1-$chloro$-2-$methylpropane as a major product
• D
  $1:9$ mixture of $1-$chloro$-2-$methylpropane and $2-$chloro$-2-$methylpropane