I haven't heard any news from him at all.

In this experiment, we will use the International System of Units.

An imaginary unit is an important concept in mathematics.

An astronomical unit represents the average distance from the Earth to the Sun.

This product has a very small assembly unit.

An identity matrix is a square matrix in which all the elements on the diagonal are 1 and all other elements are 0.

He sneaked into the house in the middle of the night.

He became good at playing the piano, and finally acquired the skill.

This book is no longer available anywhere.

He was so ashamed of his failure that he wished he could crawl into a hole.