杰克注意到他哥哥脸上焦虑的表情。

Jack ___observed a look of anxiety on his brother's face.

警卫没有注意到谁交付了包裹。

The guards failed to ___observe who delivered the package.

在社会群体之间可以观察到言语风格的差异。

Differences in speech styles can be ___observed between social groups.