The longer a text is, the more difficult it can be to identify the main idea. Strategies for identifying the main idea of a text include:
Look at the title of the text, as well as any pictures or headings included
Read the first and last sentence of the passage
Note or mark any words or phrases that are repeated throughout the text
Summarize the text as best you can in one sentence (so before looking at the answer options)
Ask yourself – what was this text mostly about?
Finally: what does the writer want me as a reader to know/think/realise about this topic?