下列关于女性月经周期的叙述中，正确的是（　　）​

    People can often see a talking parrot on a TV show, in a movie, or even in someone's home. The parrot has learned ____ (copy) sounds that people make. Dolphins, bats, and some other animals also copy sounds. Now we can add elephants ____this list of copycats (盲目模仿者).

    Dr. Joyce Poole is an expert,____studies the sounds of elephants. While she was in Kenya, she would hear strange noises ____ (make) by Mlaika after sunset. Mlaika was____ 8-year-old African elephant and it lived near a highway.

    Dr. Poole says that she couldn't tell the ____ (different) between Mlaika's call and the ____ (distance) truck noise. Why did it copy the sounds of the trucks driving by? Animals that are able to copy sounds may enjoy ____(practice) new sounds. When they ____ (keep) outside of their natural environment, they may copy unusual sounds. 

    So far Dr. Poole ____(spend) 18 years with two female Asian elephants. Asian elephants make sounds like birds to talk with one another.

    Parrots, dolphins, humans, and elephants show that being a copycat is one way that animals and people make new friends and keep old ones.