Paragraph Two A sand castle is made of a network of sand grains glued together by very thin bridges of water. Without any water, sand flows; a perfectly dry sand castle would collapse into a heap. Too much water, on the other hand, creates sand soup. But embedded in a matrix of water molecules, sand particles stay in place, somewhat like molecules in solid substances. 37. Proper amount of water is the key to b____ sand castles.
【正确答案】：build
【题目解析】：题干主要描述了沙是怎样筑成城堡的，横线上缺少的是建筑的意思，因此是build。
