英语高手请进，一个理解问题

Some of the world’s finest stone architecture can be seen in the ruins of the ancient Inca city of Machu Picchu high in the eastern Andes Mountains of Peru. The “doorways and windows” are made possible by placing over the open spaces thick stone beams that support the weight from above. A structural invention had to be made before the physical limitations of stone could be overcome and new architectural forms could be created. That invention was the arch, a curved structure originally made of separate stone or brick segments.

问：作者为什么提到“doorways and windows”，高手们觉得是为什么？我就不把选项和自己的理解写出来了，以免混乱大家的思路

好吧，大家都没有答题方向了，我还是把选项写出来吧

○To indicate that the combined skeletons and skins of the stone buildings of Machu
Picchu were similar to igloos and adobe structures
○To indicate the different kinds of stones that had to be cut to build Machu Picchu
○To provide an illustration of the kind of construction that was required before arches
were invented
○To explain how ancient builders reduced the amount of time necessary to construct
buildings from stone.

是不是指的是拱门啊？呵呵 个人理解

1.left one 2i think so ,using scientific way to keep fit can improve life expectency,but the bad factors can shoten it.

My family I have a happy family. There are four people in it. They are my father, my mother, my brother and I .My father is a doctor and my mother is a teacher. My brother and I are students. At weekends, we often go to the park. My parents love us very much. We love them too. This is my family. A sweet family. 我有一个快乐的家庭。里面有四口人，爸爸，妈妈，哥哥和我。我的爸爸是个医生，妈妈是个老师，哥哥和我都是学生。在周末，通常会去公园。我的父母很爱我们，我们也爱他们。这就是我的家，一个甜蜜的家。