In this task, you are given a context and four options. Each option is a suggested ending for the context. You should read the context and pick the best ending for the context. Please answer with "A", "B", "C", and "D".

[header] How to take a horse's vital signs [title] Use a thermometer. [step] Lubricate the tip with petroleum jelly or vaseline. [title] Move the horse's tail to the side, so that it is out the way. <sep> (A) [title] Put the thermometer tip on the horse's backbone (the thickest part, usually your horse's neck). [title] Slide the thermometer gently into the horse's mouth. (B) [title] Hold the lead at the nape of the neck with both hands and look for signs of weakness, tenderness, sudden discomfort, etc. [step] Release the lead and stop moving a few feet away from the horse. (C) [step] You want to make sure the thermometer is inserted carefully and closely. The thermometer should be able to see the internal temperature of the blood and not just the body temperature. (D) [step] Insert the thermometer into the horse's rectum, angled slightly towards the ground. [title] Leave the thermometer in position for at least three minutes or until it beeps.
D