This problem is very difficult.

It's difficult to remember the units of weights and measures.

It took him time to recall the poem.

What is the unit of measure for this product's weight?

I feel like I remember seeing that person somewhere.

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

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

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.