---- I am not able to solve some practical problems using Newton’s First Law of Motion. ----That’s just _______ most of the students are puzzled easily.

I lost a book, ______I can't remember now. 

   ______they have been working hard to find?

I try to understand __________ that prevents so many Chinese from being as happy as one might expect.

Scientists generally agree that Earth's climate will warm up over the next 50 to 100 years ________ it has warmed in the 20,000 years since Ice Age.