What role did Mencius and Xin Zi play in the life and legacy of Confucius?
A. These philosophers added to Confucius's ideas and help them spread across China.
B. These philosophers worked against Confucius's ideas centuries after his death.
C. These philosophers competed with Confucius during his lifetime and tried to promote their own ideas.
D. These philosophers worked with Confucius during his lifetime and helped him develop his ideas.