A、Branches. B、Grasses. C、Mud. D、Straw. C第3种鸟是oven bird，而题目询问灶巢鸟的窝是用什么做的。关键是要听到“这个窝是用泥巴做的。”故C正确。此外，原文提到Oven Bird时，三次提及了mud这个单词，应该

Did you know that there’s a kind of bird that can sew? This bird, called the "Tailor Bird", uses its mouth as a needle. [19]It sews leaves together in the shape of a cup. Then it adds a layer of straw to the inside of the cup and lays its eggs there. Each bird species builds its own special kind of nest. The most common materials used for nests are grasses, branches, and feathers. A bird must weave these materials into a nest. Just imagine building a house without cement or nails to hold it together! Another bird is called the "Weaver Bird". The "Weaver Bird" builds a nest that looks like a basket. The nest does shape like a pear with a hole in the middle. [20]The hole is the door of the nest. A third bird is called the "Oven Bird". The "Oven Bird" makes a nest that is very solid.
    [21]The nest is made of mud. The "Oven Bird" forms the mud into the shape of an oven, and then lets it dry in the sun. The sun bakes the mud, making it very hard. Not all birds make their homes in branches. [22]Some birds build their nests on the ground while others bury their eggs under the ground, and some birds do not build nests at all. So when you look for nests and eggs in the branches of trees and bushes, [22] remember that some nests may be right under your feet.
19. What does the nest built by a Tailor Bird look like?
20. Why is there a hole in the Weaver Bird’s nest?
21. What is the Oven Bird’s nest made of?
22. What might surprise us about birds’ nests according to the speaker?