婚礼的具体日期仍然是个秘密。

The exact date of the wedding ___remains ___a ___secret.

宝藏的真正位置仍然是个谜。

The true location of the treasure ___remains ___a ___mystery.

他突然离开的原因仍然是个谜。

The reason for his sudden departure ___remained ___an ___enigma.