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【单选题】
The best definition of 'biosphere' as it is used in the text is ______.A.
the protective envelope formed by the atmosphere around the living earth
B.
part of the earth and its atmosphere in which life can exist
C.
the living things on earth whose existence is made possible by circulation of the atmosphere
D.
the circulation of the atmosphere's contribution to life on earth
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【单选题】在使用css3盒模型时，box-sizing属性设置为（），元素的宽度只是该元素内容的宽度，而不包括边框和内边距的宽度
A.
content-box
B.
border-box
C.
text-box
D.
none