【原创】The first theories and experiments with cloning began in the late 1880s as scientist sought to prove their theories about how the genetic material inside cells worked. Discoveries about the nature of DNA in the 1940s made it possible for cloning experiments to progress. In 1944 it was discovered that genetic information for each cell was kept in the cell’s DNA. When Oswald Avery found this genetic information, it gave scientists new ways to try to clone animals by using that genetic blueprint（生物细胞的模型）.
The first cloned animals were northern leopard frogs that were cloned by Thomas J. King and Robert Briggs in 1952. King and Briggs believed, based on their clones, that young cells were more practical for the cloning process. Cells that were taken from adults resulted in abnormally developed tadpoles(蝌蚪). The next successful cloning experiments also resulted in cloned frogs. John Gurdon cloned South African frogs in 1962. His use of adult cells disproved the previous theory that only young cells could be used in the cloning process with success.
While animal cloning had been the focus of cloning experiments, the 1960s also saw other types of cloning. In 1964, F. C. Steward took an adult root cell from a carrot plant and successfully cloned the plant. Throughout the rest of the 1960s, scientists continued to clone frogs and to discover more about DNA. The first gene was discovered in 1969.
In 1977, the first cloned mice embryos were created. The first mammal was cloned in 1984. The cloned sheep was quickly followed in 1985 with cloned cattle embryos. A cow clone was created in 1986 and several calves in 1993. That same year, human embryos were cloned for the first time. In 1995 and 1996, sheep were cloned, including the famous Dolly.
The first paragraph serves as a(n) __________________________.

A．explanation	B．conclusion
C．comment	D．background

When as the first plant successfully cloned?

A．In 1952	B．In 1960
C．In 1964	D．In1969

How is the passage organized?

A．In order of frequency

B．In order of time

C．In order of importance

D．In order of preference