OG155关于that是否省略的疑问

155. Federal authorities involved in the investigation have found the local witnesses are difficult to locate, reticent, and are suspicious of strangers.
(A) the local witnesses are difficult to locate, reticent, and are
(B) local witnesses to be difficult to locate, reticent, and are
(C) that local witnesses are difficult to locate, reticent, and
(D) local witnesses are difficult to locate and reticent, and they are
(E) that local witnesses are difficult to locate and reticent, and they are

OG的解释中有一句: Moreove, A,B, and D lack the conjunction that needed to introduce the direct object clause.

OG的意思是that引导直接宾语从句所以应该保留.但是,that只起连接作用,并没有在从句中充当任何成分,按理可以省略.想不明白,请NN请教!

宾语从句中that一定不可以省略