单选题 Game fish cannot live in lakes that become filled with mud and choked with weeds. In order to restore such lakes to their original free state, a new type of dredge has been developed. This huge dredge, which costs less than earlier dredging methods, chews up the weeds, sucks up the bottom mud and muck into a floating pipe, and discharges the debris on the shore. This dredge is called the world's largest vacuum cleaner since, like a household vacuum cleaner, its operating principle is ______