Use the correct part of speech and complete the passage. A final broad a. (classify) which is sometimes used to b. (category) nouns is the c. (distinct) between variable and non variable nouns. Variable nouns are those which have d. (single) and a plural form, whereas invariable nouns don't. So count nouns are variable but mass nouns aren't. However, there are some irregularities dotted throughout the system. A few count nouns, such as sheep and deer, have no actual marker of e. (plural) and a plural sense.