用于贸易结算及优于0.5级的超声波流量计检定周期为年（）

A. 1

B. 2

C. 3

D. 4

此题为判断题(对，错)。

A、准确度高（0.5级），能满足天然气贸易交接计量的要求High accuracy (0.5 level), can meet the requirements of natural gas trade handover measurement

B、适用的流量范围大Applicable flow range is large

C、直管段较短，节省占地面积The straight section is shorter, saving the floor space

D、由于气体超声波流量计无移动部件，无压力损失，无磨损，无潜在泄漏，因此维护量小。Because the gas ultrasonic flowmeter has no moving parts, no pressure loss, no wear, no potential leakage, so the maintenance amount is small

简述DANIEL超声波流量计的工作原理。