It is essential that the glass ______ thicker on the outside than on the inside and that t
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It is essential that the glass ______ thicker on the outside than on the inside and that the gap between the panes of glass______ a minimum of 7 centimeters.
A．is, is
B．be, is
C．be, be
D．is, be
请帮忙给出正确答案和分析，谢谢！