Final answer:
To find a sentence that features an introductory phrase that explains why the main action happened, we should look for a sentence that establishes a cause-and-effect relationship, setting up the context for the ensuing action.
Step-by-step explanation:
The question is asking which sentence serves as an introductory phrase that explains the reason behind the occurrence of the main action. Introductory phrases set the stage for the main action by providing necessary context, such as a cause-and-effect relationship, background information, or setting up the significance of the action to follow. When we look at the options provided, we should identify a sentence that offers a causal relationship or explanatory detail that precedes the primary action or argument presented in the text.
For instance, if we take the working topic sentence 'Because (cause), (result)', we are looking for a sentence that fits into the 'Because' part of this structure. It's the cause that triggers the effect, which is the main action mentioned in the subsequent sentence(s). Therefore, an introductory sentence with this structure will fulfill the criteria of explaining why the main action happened.