一旦冷却，熔化的玻璃就会变成固体。

Once cooled, the molten glass becomes ___solid.

说明书上写着要将甜点冷藏至完全凝固。

The instructions say to chill the dessert until it is completely ___solid.

煤炭作为一种固体燃料，在历史上对工业至关重要。

Coal, a ___solid fuel, was historically vital for industry.