什么是主题词？
参考答案：主题词是指能够准确表达公文主题内容便于公文检索查询的规范化名词或名词性词组。
若载重3t的卡车有x辆，载重5t的卡车比它多4辆，它们一共运货yt，那么x、y的关系式为（ ）。
Look! Tina is ___________ in the lake and her parents are ___________. [ ]
A. swims, boat
B. swiming, boating
C. swimming, boating
