Поставьте общий вопрос к предложениям.
1. Many ex-soldiers are ill
2. The cause of Gulf War Syndrome remained a mystery
Постройте отрицание
1. Many ex-soldiers are ill
2. The cause of Gulf War Syndrome remained a mystery

1. Are many ex-soldiers ill?2. Did the cause of Gulf War Syndrome remain a mystery?1. Many ex-soldiers aren't ill.2. The cause of Gulf War Syndrome didn't remain a mystery.