To understand the grammar of the sentence, you must break it into parts. A．down B．up C．off D．out 题目和参考答案——青夏教育精英家教网—

     Today we tell you about some new understanding of why cutting the main branch of a plant or tree
can lead to better development.
     The findings are from researchers on two_continents.Professor Prezemyslaw Prusinkiewicz of the
University of Calgary in Canada led the research with scientists from Britain and Sweden.Their study
appears in the Proceedings of the National Academy of Sciences.
     Researchers have known since the 1930s that the actively growing tip of a plant produces a hormone(荷尔蒙) called auxin(生长素). This hormone flows down the main stem.Scientists say the auxin has
an indirect effect on buds(芽) on the side of the stem to prevent branching.
     These buds themselves also produce auxin.The research suggests that to grow, they have to be able
to export the hormone into the main stem.But the flow from the stem tip prevents them from doing this.
The researchers wanted to find out how this blocking happens.
     Professor Prusinkiewicz is on leave in Australia, but he sent us an email suggesting a simple way to
underst and the process.Think of a major road crowded with traffic.So many cars are on the main road
that the cars on the side roads cannot enter.
      The stem is like the crowded main road.The new research shows that the buds on the side cannot
export their auxin into the main stem because it is too full.But if that main shoot is pruned(修剪), other
buds below it can start exporting.They are no longer prevented from growing.
     Ottoline Leyser from the University of York says that after a plant is pruned, all the inhibited(被抑制的) shoot tips compete with each other to grow.In doing this, the branches influence each other's growth.
Nearby shoot tips are more likely to affect each other than those that are far apart from each other.
      Professor Leyser says the strongest branches grow best, wherever they may be on the plant.The
study found that the main shoot grows the best of all not because of its position at the top of the plant,
but mostly because it got there first.

1. The underlined part "two continents" in Paragraph 2 refers to "________".
A. Asia and Africa
B. Africa and Europe
C. North America and Europe
D. Australia and Europe

2. Professor Prusinkiewicz compares the cars on the side roads to ________.
A. plant stems
B. plant buds
C. the leaves of a plant
D. the roots of a plant

3. To make the buds on the side of the stem to grow better, the main shoot should ________.
A. be cut off
B. be protected
C. be watered as much as possible
D. be given much auxin

4. According to the passage, which of the following is TRUE?
A. Researchers have recently found out that the growing tip of a plant produces hormones.
B. Auxin has a direct effect on buds to keep branches from growing.
C. Buds on the side of the stem can't produce hormones.
D. In order to grow, the buds on the side of the stem export the hormones into the main stem.

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