关于OG13 从句that的疑问

刚刚开始准备考G，用的13th，虽然可能前面版本也这样解释的

The results of two recent unrelated studies support the idea that dolphins may share certain cognitive abilities with humans and great apes; the studies indicate dolphins as capable of recognizing themselves in mirrors-an ability that is often considered a sign of self-awareness-and to grasp spontaneously the mood or intention of humans.
   A.dolphins as capable of recognizing themselves in mirrors-an ability that is often considered a sign of self-awareness-and to grasp spontaneously
   B.dolphins' ability to recognize themselves in mirrors-an ability that is often considered as a sign of self-awareness-and of spontaneously grasping
   C.dolphins to be capable of recognizing themselves in mirrors-an ability that is often considered a sign of self-awareness-and to grasp spontaneously
   D.that dolphins have the ability of recognizing themselves in mirrors-an ability that is often considered as a sign of self-awareness-and spontaneously grasping
   E.that dolphins are capable of recognizing themselves in mirrors-an ability that is often considered a sign of self-awareness-and of spontaneously grasping

我是直接用平行的方式选的，但是看后面解释的时候，它说“the studies indicate must intorduce a clause; the clause must begin with that and have a subject".
于是有点疑惑了，它这个解释是说从句后面都必须要跟that么，我查找了以下Topway上门之前的一些讨论，有说是say／announce后面是可以省略的，现在仍然是这种说法么？？
无比感谢啊。。。。为菜鸟解下惑吧。。。。