A child is big enough to use the vehicle lap and shoulder belt when:

• They can touch the vehicle ceiling with their head.
• They can sit all the way back with their knees bent at the edge of the seat.
  Explanation:

  "A child is big enough to use the vehicle lap and shoulder belt when (1) they can sit with their back against the vehicle seat's back and (2) their knees bend over the edge of the vehicle seat."
• Their knees cannot easily bend over the edge of the seat.
• Both B & C