The bottom is a square with a side length of 4, and the four vertices are a, b, c and d from the upper left counterclockwise. Then on the edge CD, take the distance from point E to point C 1, that is, CE= 1 and ED=3. Then make a line segment EO perpendicular to the square ABCD from point E, so that EO=4. Connect o with the vertices of the square. O-ABCD(COD surface is perpendicular to the bottom; The other three faces are not vertical, but inclined).

Glue the bottom of this cone, that is, one side of the square, to a transparent glass. We lift the glass so that the cone is on both sides of the glass with us (look at the four points of the square through the glass, and look at A, B, C and D clockwise from the bottom left). )。 At this time, starting to look at this cone from the front, left and top is the three-view effect in the topic.

The longest side length is side A0. Connect EA and EO. Because the triangle ADE is a right triangle, and AD=4, DE=3, AE=5. In the right triangle OAE, AE=5, EO=4? So AO=√4 1 (4 1 under the root sign), and the answer is C.