A driver is uncoupling a semi-trailer on what appears to be a solid asphalt surface on a hot day. Which of the following is a critical extra step to ensure the trailer remains stable after uncoupling?

1. Placing boards or pads under the landing gear.
2. Disconnecting the electrical line before the air lines.
3. Lowering the landing gear only halfway.
4. Chocking the tractor wheels instead of the trailer wheels.