During his military campaign in what is now Germany, Julius Caesar lead his army of 40,000 soldiers to the western bank of the Rhine River in 55 BCE. On the opposite side of the river, hostile native warriors witnessed their arrival. Caesar determined that the river was both too wide (up to 400 meters at some locations) and too deep (about 30 meters) to cross by foot. But Caesar still wanted to find a way to cross. Which step in the engineering design process is described above?