【单选题】The isometric projection of a frontal circle is an ellipse. The minor axis of the ellipse is in coincidence with , and the major axis of the ellipse is perpendicular to .
【单选题】The isometric projection of a profile circle is an ellipse. The minor axis of the ellipse is in coincidence with , and the major axis of the ellipse is perpendicular to .
【简答题】C ombining with examples, explain what is a datum , what is the design datum , technological process datum , sequential datum , locating datum, measuring datum , and assembly datum ?