Which detail from this first-hand account of the Tet Offensive demonstrates that it was a surprise attack?

See below.

Explanation:

Tet was launched in January 30th 1968, Tet being the Chinese New Year.

The attack was a complete surprise. The guard of the South Vietnamese army was lowered by Tet celebrations. Moreover the Vietcong and NVA switched their offensive from the countryside to the cities, which again took the South Vietnamese and Americans by surprise.

The fact that the Vietcong could get right inside the American embassy in Saigon emphasized the surprise nature of the attack.

Tet was actually a military defeat for the communists as they sustained massive losses. However psychologically it was a massive victory for them. Widespread coverage of the raging battles in cities such as Saigon and Hue on American televisions every night, swayed public opinion against the war, as the images suggested the Vietcong could go anywhere, and so it suggested the US was losing the war.

It's A

Explanation:

The troops expected to return to bed 15 minutes later.

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