单项选择题

Until late in the eighteenth century a soldier wounded in battle was likely to lie where he fell until nightfall or until the battle was over. Even then his chance of seeing a doctor was little unless other soldiers carried him to the back. Doctors did not usually reach the battlefield until the day after the battle.
But toward the end of that century the French army found a new way to help the wounded. It was a vehicle equipped to carry the wounded. It was equipped with hospital supplies and moved behind the battle lines to pick up and carry the wounded to hospitals. In French, it was called a hospital ambulant, or "travelling hospital". The second word came from the Latin word "ambulo", which means "travellers". In England the name was shortened to our present word "ambulance".
Doctors usually reached the battlefield ______.