单项选择题 三态输出门有3种输出状态，输出高电平、输出低电平和工作状态。 A、正确 B、错误

问答题 What may make  the international sale of goods more complex than the domestic sale? A、Different language B、Different culture C、Different currency D、Long distance of transport E、Different legal system F、Different metrology system

单项选择题 有关杂种优势，论述不正确的是：（ ）