例1 [结果]These plants are termed opportunists because [原因]they rely on their seeds’ falling into settings where competing plants have been removed by natural processes, such as along an eroding riverbank, on landslips, or where a tree falls and creates a gap in the forest canopy.
A. Because [原因]their seeds grow in places where competing plants are no longer present, [结果]dandelions are classified as opportunists.
B. [结果]Dandelions are called opportunists because [原因]they contribute to the natural processes of erosion and the creation of gaps in the forest canopy.
C. The term opportunists apply to plants whose seeds fall in places where they can compete with the seeds of other plants.
D. The term opportunists apply to plants whose falling seeds are removed by natural processes.
例2 [转折前]Estimates indicate that the aquifer contains enough water to fill Lake Huron, but unfortunately, [转折后]under the semiarid climatic conditions that presently exist in the region, rates of addition to the aquifer are minimal, amounting to about half a centimeter a year.
A. Despite the current impressive size of the Ogallala aquifer, the region’s climate keeps the rates of water addition very small.
B. Although the aquifer has been adding water at the rate of only half a centimeter a year, it will eventually accumulate enough water of fill Lake Huron.
C. Because of the region’s present climatic conditions, water is being added each year to the aquifer.
D. Even when the region experiences unfortunate climatic conditions, the rates of addition of water continue to increase.