重点讲解：
1. at present 目前；现在；眼下；
The grotto is not accessible at present.
该石窟目前不可进入。
2. make up 组成；构成；
North Africans make up the largest and poorest immigrant group in the country.
北非人构成了该国最大的、也是最贫困的移民群。
3. let one's guard down 丧失警惕；放松警惕；不提防；
You can't let your guard down.
你不能放松警惕。
4. due to 由于；因为；
The train had stopped due to a mechanical problem.
火车因为机械故障停了下来。