As we bumped over the dusty track, we swerved to avoid large boulders. The wheels scooped up stones which hammered ominously under the car. We felt sure that sooner or later a stone would rip a hole in our petrol tank or damage the engine.

Because of this, we kept looking back, wondering if we were leaving a trail of oil and petrol behind us.

1) 这里的leave是静态动词，动态动词，还是短暂动词？使用进行时leaving是表示反复发生的动作吗？对比如使用一般过去时left的含义会怎样？（wondering if we LEFT a trail of oil and petrol behind us.）

What a relief it was when the boulders suddenly disappeared, giving way to a stretch of plain where the only obstacles were clumps of bushes. But there was worse to come. Just ahead of us there was a huge fissure.

2) 黑体字it形式主语指代的是整个when the boulders... clumps of bushes吗？另外，在（when the boulders suddenly disappeared, giving way to a stretch of plain where the only obstacles were clumps of bushes），

giving way to 的逻辑主语是the boulders，整个when从句(一直到bushes)是修饰前面的感叹句What a relief it was ?

In response to renewed pleadings, Bruce stopped. Though we all got out to examine the fissure, he remained in the car. We informed him that the fissure extended for fifty years and was tow feet wide and four feet deep. Even this had no effect. Bruce went into a low gear and drove at a terrifying speed, keeping the front wheels astride the crack as he followed its zigzag course. Before we had time to worry about what might happen, we were back on the plain again. Bruce consulted the map once more and told us that the village was now only fifteen miles away. Our next obstacle was a shallow pool of water about half a mile across. Bruce charged at it, but in the middle, the car came to a grinding half. A yellow light on the dashboard flashed angrily and Bruce cheerfully announced that there was no oil in the engine!

3) In response to renewed pleadings 介词短语作非谓语表原因？这里的as he followed its zigzag course是修饰keeping the front...还是修饰前面的谓语drove at...呢？

谢谢老师！