The passage is a piece of ______.

A．older adults need more physical exercises than the younger ones
B．younger adults are not necessary to take a physical examination before a fitness program
C．fitness programs for older adults should be well-monitored
D．poorly-designed fitness programs for older adults will do serious harms to them

A．producers can satisfy the consumers by mechanized production
B．consumers can express their demands through producers
C．producers decide the price of products
D．supply and demand regulate price