The core concept is to understand the meaning of 'fixed expressions' which are phrases or groups of words that are used in a specific order and cannot be altered without sounding wrong. * **Correct Answer:** C. 'Set phrases' means a group of words used together frequently with a particular meaning, which is the same concept as 'fixed expressions'. * **Incorrect Options Analysis:** * A. 'Optional phrases' implies phrases that are not required, which does not correlate with fixed expressions. * B. 'Flexible statements' are phrases that can be changed and adapted. The idea is opposite of 'fixed'. * D. 'Variable idioms' implies phrases whose meaning is not literal and can vary. 'Fixed' means the opposite of 'variable' **Key Point**: The word 'fixed' indicates that these phrases are not changeable or variable. **Common Misconception**: Students might confuse 'fixed' with 'optional', 'flexible', or 'variable', failing to identify the term's indication that the expression must be used in its prescribed form.