(20 13? Tianjin) has known the following three propositions: ① If the radius of a ball is reduced to the original, its volume will be reduced to the original; (2) if there are two sets of numbers

① According to the volume formula V= of a ball, if the radius of a ball is reduced to the original, its volume is reduced to the original; Therefore, ① is correct;

② If the average value of two sets of data is equal, their standard deviations are not necessarily equal, such as 2,2,2 and 1, 2,3; The average value of these two sets of data is equal, but their standard deviations are not equal, so ② is wrong;

③ The distance from the center of the circle to the straight line x+y+ 1=0 is d= = radius r, so the straight line x+y+ 1=0 is tangent to the circle, and ③ is correct.

So choose C.