The unit of length most suitable for measuring the thickness of a cell phone is a . the unit of length most suitable for measuring the height of a backyard tree is a .

Millimeter and meter

Explanation:

Cellphones are only a few millimeters in thickness.

It is easier to say 5 millimeter than 0.0005 meter.

The unit we use must help others easily visualize and understand the size of the object.

a 5 millimeter is easy to visualize than a 0.0005 meter or a 5,000,000 nano meter

In the same way, a tree in our backyard usually grow as tall as a few meters. ( remember trees in amazon forest might grow as tall as a few kilometers). so in the case of trees in our home the right unit to measure them is meters.

the answer is neither ball has a charge.

because if they were both positively charged, they will repel each other.

if they were positively and negatively charged, they will seek.