What rule applles to this word?
exhaust
Group IV. When the root has other forms built on the letter a, the able form is used.
Group VII. When the word has an immediate -tion form, -ible is used.
Group V. When the root ends in hard cor hard g. able is used.
Group II. When the root is a full word except for a final e, the forms usually are spelled with able
Group VI. When the root is not a full word, ible is used.
Group VIII. When the root ends in ns, miss, or soft cor g, ible is used.
Group I. If the root is a full word, the ending is usually able.
Group Ill. The suffix -able always follows