良好的面向对象程序设计风格主要有哪些准则？
参考答案：提高可重用性、提高可扩充性、提高健壮性。
My sister _____ the league last month and she has decided to ______ the activities to help the aged in
town next week.
[ ]
A. joined; join in
B. joined in; join
C. attended; join
D. took part in; join in
简述链球菌的主要致病物质。
