______ no conclusive evidence exists, many experts believe that the wheel was invented only once and then diffused to the rest of the world.

A．it helps strengthen our ties with relatives
B．it enables us to eliminate our faults and mistakes
C．it makes our leisure-time activities more enjoyable
D．it draws our attention away from our worries and troubles

A．relies on the social welfare systems which support them
B．has much to do with the amount of support they get from others
C．depends on their ability to deal with daily worries and troubles
D．is closely related to their strength for coping with major changes in their lives