A、To give them fruits. B、To provide water. C、To plant flowers. D、To keep pets. B短文谈到，其中一个最确定的方法是提供水，故选B。

One of the great joys of gardening is to share a garden with wildlife, especially birds. Attracting birds to a garden is made easier by following a few basic guidelines.All birds depend on plants for shelter, nest and roosting sites, and food. To the extent that your yard offers some or all of these features, it can become a heaven for birds’ life. As a general rule, the more native plants are planted in the yard, the more birds will come. Some birds are so particular that you must have the special plant that they demand while others only demand that you provide their basic needs. Most birds require some shelter for roosting, preening and sleeping. By varying the height as well as the density of plants, a greater number of species are encouraged to seek out shelter in your yard. Nesting requirements in birds are quite varied.To encourage as many different types of birds as possible, vary the types of plants, use a number of native plants, and vary the height of plants. Birds eat seed, fruits, or nectar, as well as insects found on plants. Again, diversity encourages more birds, particularly plants that bloom in different seasons. One of the surest ways to encourage birds in a yard especially a new yard, is to provide water. If you have pets or wandering cats, move the water higher, beyond their reach. Leave the water in the open, with no overhanging branches. Change the water often, especially in the warm weather, and remove algae buildup with a scrub brush rather than chemical treatments.
20. According to the speaker, what are the general requirements to attract birds?
21. How tall should plants be in order to attract birds?
22. What is one of the surest ways to encourage birds in a yard?