第 44 至 47 題為題組         Spider webs are one of the most fascinating examplesof animal architecture. The most beautiful and structurallyordered are the orb webs. The main function of the web isto intercept and hold flying prey, such as flies, bees andother insects, long enough for the spider to catch them. Inorder to do so, the threads of the web have to withstand theimpact forces from large and heavy prey as well asenvironmental forces from wind and rain for at least a dayin most cases.
         The orb web is found to have two main characteristics. The first is its geometry, which consists of anouter frame and a central part from which threads radiate outward. Enclosed in the frame are capturespirals winding round and round from the web center out to the frame. The whole web is in tension andheld in place by anchor threads, which connect the frame to the surrounding vegetation or objects. Thesecond and perhaps most important characteristic is the material with which it is built. Spider silk is akind of natural composite that gives this lightweight fiber a tensile strength comparable to that of steel,while at the same time making it very elastic. Two types of silk threads are used in the web. One is highlyelastic and can stretch to almost twice its original length before breaking and, for most types of spiders, iscovered in glue. This type is used in the capture spiral for catching and holding prey. The other is stifferand stronger, and is used for the radius, frames and anchor threads, which allows the web to withstandprey impact and to keep its structural strength through a wide range of environmental conditions.

【題組】44. What is this passage mainly about?
(A) The food network in nature.
(B) The construction of orb webs.
(C) The network of geometrical studies.
(D) The environmental challenges for spider webs.