The key concept here is understanding what constitutes a 'fixed phrase' or a common collocation. * **Core concept:** A fixed phrase is a group of words that are habitually used together and have a specific meaning that may not be obvious from the individual words. * **Correct Answer (B):** Option B, 'at the same time,' is a common fixed phrase used to denote simultaneous events or actions. The phrase is commonly encountered and used with this exact structure. * **Option A:** While 'a sudden storm' is a common phrase, it doesn't meet the criteria of being a fixed expression. Each word conveys its literal meaning. * **Option C:** 'She walked quickly' is a sentence and an example of an adverb describing a verb. This is not a fixed expression. * **Option D:** 'Very beautiful flowers' is a descriptive phrase where each word conveys its individual meaning. It is not a set phrase like the correct answer. * **Key Takeaway:** Fixed phrases are frequently used combinations of words that typically retain their structure and meaning.