A circle is a circle whose center is point O and whose radius is 1
The straight line y=kx+2 intersects the y axis at point a (0 0,2).
Take two extreme values of k to make the straight line tangent to the circle,
Two values of k make a straight line up and a straight line down. So k must be a continuous interval, and both b and d are wrong.
X 2+y 2 =1and y=kx+2 intersect at point b, which is in a quadrant or a quadrant, so the triangle OAB must be an RT triangle because the straight line is tangent to the circle. According to Pythagorean theorem, the other side is the root number 3.
The answer is C.