The word "it" (Line 2, Para. 2) most probably refers to ______.
A. economic depression
B. job insecurity
C. birth rate
D. financial unsafety
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In the third paragraph, bricks are mentioned to indicate how ______.
A. science is more than a collection of facts
B. scientific experiments have led to improved technology
C. building a house is like performing experiments
D. mathematicians approach science
政府采购合同履行中，采购人需追加合同标的相同的货物、工程或者服务的，在不改变原合同其他条款的前捉下，可以与供应商协商签订补充合同，追加与合同标的相同的货物、工程或服务，所有补充合同的累积增加的、采购金额不得超过原合同采购金额的()。
A. 10％
B. 15％
C. 20％
D. 25％.
The following factors tend to promote social change except ______.
A. different kinds of people
B. different points of view
C. different interests
D. different conditions
The major problems with America's railroad system in the mid 19th century lay in ______.
A. limited railroad lines
B. lack of a transcontinental railroad
C. lack of financial support of development
D. poor quality rails and unreliable stopping systems
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