He does help others, but he always expects returns ____. As a result, nobody will accept anything from him. 

Before making your speech, you’d better ________ your thoughts and ideas.

AsＩ ____the house ,I noticed a light on upstairs, which made me quicken my pace towards the doorway.

While listening carefully,  the students＿＿＿ what their teacher said. 

Please tell me how global warming _____. I am still in the dark.