Teacher:Given a passage and a question, find the answer to the question based on the passage. The answer should consist of only one word that lies in the span of only one of the sentences in the passage.
Teacher: Now, understand the problem? Solve this instance: Paragraph: Aqueducts moved water through gravity alone , along a slight overall downward gradient within conduits of stone , brick , or concrete ; the steeper the gradient , the faster the flow . Most conduits were buried beneath the ground and followed the contours of the terrain ; obstructing peaks were circumvented or , less often , tunneled through . Where valleys or lowlands intervened , the conduit was carried on bridgework , or its contents fed into high - pressure lead , ceramic , or stone pipes and siphoned across . Most aqueduct systems included sedimentation tanks , which helped reduce any water - borne debris . Sluices and castella aquae ( distribution tanks ) regulated the supply to individual destinations . In cities and towns , the run - off water from aqueducts scoured the drains and sewers .
Question: what force did the romans use to move water

Student:
gravity