One way in which exercise can improve our health is that it helps us to________.

A． talks as much as participants.
B． is usually a "constant talker".
C． prefers to take the role of an observer.
D． is frequently outshone by participants.

A．expand and improve the existing subway system.
B．build underground malls and parking lots.
C．prevent further land subsidence.
D．promote advanced technology.