请选择DCBA7L+r,S(v.E7R5C3C/s Helen is good _________ swimming.-s1t4~4C-r;N9|
A. on-s1t4~4C-r;N9|
B. in-s1t4~4C-r;N9|
C. at-s1t4~4C-r;N9|
D. to-s1t4~4C-r;N9|