Watermelons have been cultivated for more than 4000 years and thumping themto test for ripeness has an equally long history.这就是一个由并列连词 and 连接的简单的并列句，第二个分句的主语是动名词短语 ：thumping themto test for ripeness（敲瓜判断是否成熟）；whereby：凭借、通过......方式 ，诚如楼主所判断，填B,句子结构虽然无误，逻辑意义上却说不通。 顺便说一下，whereby既可作副词，也可作连词，故完全可以接句子：
For example , govern...
whereby= by means of which 是个关系代词，用来引导定语从句，属于正式用语，意思是“凭借......”在从句中作方式状语。例如：You have to devise a means whereby you can settle the problem.
你必须设计一种方法，用它解决这个问题。楼主...
