The first method: please draw your own picture. There is a in the upper left corner, followed by ABCD clockwise.

So AD =2, DC=4 and BC =3.

According to the meaning of the question, it is easy to know that the right angle of the triangle must be left, so the other right angle in the picture is cut out.

1. Let ∠D be the right angle of the original triangle, then A and C are the points on the side of the right angle, and B is the midpoint of the hypotenuse.

According to the nature of a right triangle, the length of the hypotenuse is equal to twice the center line of the hypotenuse.

The length of the hypotenuse L=2BD. Using Pythagorean theorem, it is easy to get BD=5.

So L= 10

2 .. Let ∠C be the right angle of the original triangle, then B and D are the points on the side of the right angle, and A is the midpoint of the hypotenuse.

be the same as the above

The length of the hypotenuse is L=2AC, and AC=2√5, which can be easily obtained by Pythagorean theorem.

So L=4√5

To sum up, the length of the hypotenuse is 10 or 4√5.

C

The second method (this method is only for entertainment, and it will be a blow in an emergency):

Some types of math multiple-choice questions can be observed at a certain time, and the answers can be observed directly under the premise of insufficient time.

For example, the answers to options A and B belong to option C, options C and D have two answers and both have the answer 10, while 2√ 17 only appears once, 4√5 twice and 10 three times.

So I might guess the answer C by luck.