题干

当物体距凸透镜20cm时,在透镜的另一侧的光屏上得到一个____(填“正立”或“倒立”)的、等大的清晰的像,则此凸透镜的焦距为____cm,此时像距凸透镜____cm.若将此物体向透镜靠拢,而在屏上仍能得到清晰的像,则光屏应该向____凸透镜的方向移动,在此过程中,像逐渐变____.当物体移到离凸透镜10cm时,无论怎样移动光屏也不能在光屏上得到清晰的像,通过凸透镜____(填“能”或“不能”)看到物体的像.

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倒立,10,20,远离,大,不能

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     Butterfly gardens can be grown throughout the United States.There is a wide variety of both butterfly­attracting (nectar) plants and host plants covering climate zones throughout the country.Butterfly gardens can range  in size from a few containers placed in a sunny spot to several acres.

     Nectar­producing (产蜜的) plants will attract butterflies to your garden.In order to support a full butterfly life cycle,host plants (for laying eggs and used as a caterpillar food source) must also be present.

     Throughout the country,the general requirements  for butterfly gardening are the same:full sun,nectar source plants,larval (幼虫的)  host plants,a pesticide­free environment,and knowledge of the local butterfly  species.Many butterfly­attracting plants are natives and require little attention,as they are naturally adapted to the region in which they live.

     Butterfly gardens are best planted in spring with younger plants or in fall with mature plants that will become dormant quickly and re­emerge in  spring.It is best not to plant in the heat of summer or the cold of winter.

     One of the most common mistakes in butterfly gardening is planting only one nectar source.Adult butterflies have a very short lifespan.Planting a variety of nectar sources will encourage more butterflies to visit the garden.Planting an adequate supply of host plants gives butterflies a place to lay their eggs,which will successfully hatch (孵化) and result in butterflies that will continue to visit the garden.

     Butterflies typically lay their eggs in late spring and hatch 3-6 days after they are laid.It takes 3-4 weeks for a caterpillar to pupate (化蛹) and 9-14 days to emerge as an adult.

      Good resources for learning more about butterfly gardening include zoos,botanical gardens,butterfly houses,garden centers and nurseries,libraries and the Internet.There are many good websites dedicated to butterflies and butterfly gardening.