Such acts of sabotage are particularly difficult to prevent because they do not become evident until the trigger is activated—-by remote control.
【正确答案】：这是一个由because引导的原因状语从句；not... until...指直到……才；remote control指远程控制。答案为：这种蓄意破坏特别难以防备，因为直到通过远程控制激活，这种破坏程序才显现出来。
