大全939新问，状语从句中的主谓省略

andy_lxh

939．    With its abundance of noun inflections, Icelandic is one of several Germanic languages that is compact when written but can lengthen considerably when translated into English.
(A) is compact when written but can lengthen considerably when translated into English
(B) are compact when they are written, but they can lengthen considerably when they are translated in English
(C) is compact when written but can lengthen considerably when being translated into English
(D) are compact when written but can lengthen considerably in English translation
(E) is compact when it is written but can lengthen considerably when translated in English

答案选D

(D) are compact when written but can lengthen considerably in English translation

请教各位大神, that 从句主谓是 are compact , be+adj, 但When从句 省略主谓when(they are) written为 Be+ done， 前者系表结构，Be为系动词， 后者Be助动词，循例应该不能省略， 此处如何理解