[单项选择]Some people view the findings with caution, noting that a cause-an&effect relationship between passive smoking and cancer remains______
A. to be shown
B. to have shown
C. to have been shown
D. being shown

[单项选择]Some people viewed the findings with caution, noting that a cause-and-effect relationship between passive smoking and cancer remained _________.
A. to be shown
B. to have been shown
C. to have shown
D. being shown

[单项选择]

Most young people enjoy some forms of physical activity. It may be walking, bicycling, or swimming, or in winter, skating or skiing. It may be a game of some (36) -- football, hockey, golf or tennis. It may be mountaineering.
Those who have a passion for (37) high and difficult mountains are often (38) with astonishment. Why are men and women willing to (39) cold and hardship, and to take risks on high mountains This astonishment is caused, (40) , by the difference between mountaineering and other forms of activity to (41) men give their leisure. Mountaineering is a sport and (42) a game. There are no man-made roles, (43) there are for such games as golf and football. There are, (44) rules of a different kind which it would be dangerous to (45) , but it is this freedom from man-made rules that makes mountaineering (46) to many people. Those who climb mountains are (47) to use their own met
A. for example
B. then
C. perhaps
D. of course