"cu" is a pair of letters the element we call "copper".
the letter pair "cu" is not the name of an element; it’s a chemists use to represent it
a. chemical formula
b. name
c. symbol

C. Symbol

Explanation:

The letters CU is a chemical symbol used to identify the said metal.

Elements are distinct substances that cannot be split up into simpler substances. Such substances consists of only one kind of atom. The numbers of elements know till date exceeds hundred. Every matter is a combination these various elements.

In the world of science, each of the element is symbolized by a capital letter or capital letter followed by a small letter usually derived from their Greek, Latin or English names.

For copper, it is derived from a latin word Cuprum and its symbol is Cu.

1. they vary with a higher number because boiling is way hotter than melting