根据下面材料，回答题。 It is predicted that there will be 5 scientific breakthroughs in the 21st century. We'll know where we came from. Why does the universe exist? To put it another way， why is there something instead of nothing? Since the 1920s， scientists have known the universe is expanding， which means it must have started at a definite time in the past. They even have developed theories that give a detailed picture of the evolution of the universe from the time it was a fraction of a second old to the present. Over the next couple of decades， these theories will be refined by data from extraordinary powerful new telescope. We will have a better understanding of how matter behaves at the unfathomably high temperatures and pressures of the early universe. We'll crack the genetic code and conquer cancer. In 19th century operas， when the heroine coughs in the first act， the audience knows she will die of tuberculosis in Act 3. But thanks to 20th century antibiotics， the once dreaded， once incurable disease now can mean nothing more serious than taking some pills. As scientists learn more about the genetic code and the way cells work at the molecular level， many serious diseases——cancer， for one- will become less threatening. Using manufactured 'therapeutic' viruses， doctors will be able to replace cancer causing damaged DNA with healthy genes， probably administered by a pill or injection. We'll live longer （120 years?） If the normal aging process is basically a furious， invisible contest in our cells- a contest between damage to our DNA and our cells ability to repair that damage- then 21st century strides in genetic medicine may let us control and even reverse the process. But before we push scientists to do more， consider: Do we really want to live in a world where no one grows old and few children are born because the planet can hold only so many people? Where would new ideas come from? What would we do with all that extra time? We'll 'manage' Earth. In the next millennium， well stop talking about the weather but will do something about it. Well gradually learn how to predict the effects of human activity on the Earth，its climate and its ecosystems. And with that knowledge will come an increasing willingness to use it to manage the workings of our planet. We'll have 'a brain road map'. This is the real 'final frontier' of the 21st century: The brain is the most complex system we know. It contains about 100 billion neurons （roughly the number of stars in the Milky Way）， each connected to as many as 1，000 others. Early in the next century， we will use advanced forms of magnetic resonance imaging to produce detailed maps of the neurons in operation. We'll be able to say with certainty which ones are working when you read a word， when you say a word， when you think about a word， and so on. The sentence 'In 19th century operas， when the heroine coughs in the first act， the audience knows she will die of tuberculosis in Act 3' means__________. 查看材料