A和C的意思完全不一样. A是说 sb agree that (study was important but more research was needed...),这种情况下but应该改为and才对. C是说 sb agree (that study was important) but (that more research was needed...), 是agree x but y句型.
如果A中改为了and,那么语法正确,但意思为"sb同意more research was needed", C的意思是"sb不同意more research was needed".
B有一个致命硬伤,要是用B,句子就有两种理解:1)AGREE THAT 后面那整段是一个句子,都是RESEARCHER同意的内容。2)AGREE THAT the study of new treatments for heart attack patients was extremely important (到这里为止,是RESEARCHER同意的内容。) BUT…………后面是另外一个句子,不属于RESEARCHER同意的内容,是写这句的AUTHOR的意见。
C刚好弥补了这个缺陷:AGREE THAT BUT THAT 清晰明了,整句都是RESEARCHER 的意见。