Final answer:
The letters 'y' and 'z' do not have a direct commonality linguistically as 'y' can be a vowel or a consonant while 'z' is always a consonant; the common feature depends on the context such as both being variables in an equation.
Step-by-step explanation:
The question, "What do y and z have in common?" refers to their role in an equation and their classification in the alphabet. While y is a letter that sometimes acts as a vowel and sometimes as a consonant, z is always a consonant. Therefore, the correct answer is not that they are both sometimes vowels and sometimes consonants. Instead, the common aspect shared by y and z depends on the context of the question. If it is within an equation context, the commonality could be that both are variables. However, without an equation or additional context, there is no inherent linguistic or numerical relationship between y and z.