A、Its low cost of planting. B、Its richness in vitamins. C、The Irish Potato Famine. D、Its emigration to Europe. C录音最后提到土豆，并指出19世纪

Moderator:
    Hello Ladies and Gentleman, I’m glad to introduce our keynote speaker for today’s session, Dr. Jeffrey Lewis. Dr. Lewis, Professor of botany at Harvard University, has written numerous articles and books on plants for years.
    Dr. Lewis:
    Thank you for that introduction. Today I’m going to talk about the part that plants have played in our history, something most of us probably never give any thought to. Flowers began changing the way the world looked almost as soon as they appeared on Earth about 130 million years ago. Without those flowering plants, it’s likely we wouldn’t be here. They are fundamental to our existence, forming a vital part of our food supplies.
    If we look further on in time, to the year 2737 BC, we arrive at the discovery of tea by a Chinese emperor, when a leaf fell into a cup of hot water. This discovery eventually played a very important part in the history of China and the UK. For Britain, when demand for tea was high there were financial implications when a Chinese emperor declared that it could only be bought with silver which they were then forced to find overseas.
    A second influential plant emerged around 202 BC in the form of the White Mulberry. It became extremely sought after and it remained so until 220 AD because white mulberries provide food for the worm that creates silk. This luxurious material was responsible for establishing extremely valuable trade routes. And it was along these same routes that different religions made their way around the world. But, unfortunately, they also caused the spread of previously unknown diseases, as well as weapons of war such as gun powder.
    Finally, moving into more recent history, we come to the 16th century and the potato. It originated in Central America but it was the Spanish who introduced it to the European diet. There, it quickly became popular because not only did it cost little to grow, but it was also a food source that was rich in vitamins. However, it is perhaps more famous now for the potato famines of the mid 1800s in Ireland. The Irish people were so dependent on the crop that its widespread failure over this period caused the deaths of over a million people and the emigration of a further million to mainland Britain, Canada and the US.
    16.What plant is related with Britain’s overseas finding of silver?
    17.What contribution does the White Mulberry make to the mankind?
    18.According to the speaker, what made the potato more famous?