[填空题]It is difficult to decide exactly when there is enough ______ to say an animal is a new species.

[填空题]It is often difficult to decide who really deserves the credit for (invent) ______ a new device.

[填空题]It is often difficult (decide) ______ what to do.

[单项选择]It is difficult to say exactly how the music we call "rock" or "rock and roll" began. Its roots go back to many different countries and many different kinds of music and musicians.
Rock music developed mainly from the interaction of black African and white European music. The Europeans and the African slaves who came to America during the seventeenth century each had their own different kinds of music. Black Africans used heavy drumbeats, a rough singing voice, and the calling of a melody and answer of a chorus. The white Europeans, mostly English and Scots, used a stronger melody, a less heavy beat, and instruments such as guitars, horns, and fiddles. The history of rock and roll is the history of how rock evolved from these two traditions.
From the seventeenth to the twentieth century, musicians in the southern United States developed two new forms of music: the blues, usually played by blacks, and country music, played by whites. Black musicians sang the blues accompan
A. the life of Elvis Presley
B. the history of American music
C. the development of rock and roll
D. the development of black African and the white European music

[单项选择]

A deal is a deal—except, apparently, when Entergy is involved. The company, a major energy supplier in New England, provoked justified outrage in Vermont last week when it announced it was reneging on a longstanding commitment to abide by the state’s strict nuclear regulations.
Instead, the company has done precisely what it had long promised it would not challenge the constitutionality of Vermont’s rules in the federal court, as part of a desperate effort to keep its Vermont Yankee nuclear power plant running. It’s a stunning move.
The conflict has been surfacing since 2002, when the corporation bought Vermont’s only nuclear power plant, an aging reactor in Vernon. As a condition of receiving state approval for the sale, the company agreed to seek permission from state regulators to operate past 2012. In 2006, the state went a step further, requiring that any extension of the plant’s license be subject to Vermont legislature&rs
A. Entergy’s capacity to fulfill all its promises
B. the nature of states’ patchwork regulations
C. the federal authority over nuclear issues
D. the limits of states’ power over nuclear issues