题目

维生素是人体必须的营养物质。它们多数在人体内不能合成，需从食物中摄取。维生素C是我们熟悉的一种维生素，它的分子结构如右图所示。据图推算维生素C的分子式为__________，分子中碳、氢元素的质量比为__________。 答案：(1)C6H8O6  (2)9∶1 Can people change their skin colour without suffering like pop king Michael Jackson? Perhaps yes. Scientists have found the gene that determines skin colour. The gene comes in two versions, one of which is found in 99 per cent of Europeans. The other is found in 93 to 100 per cent of Africans, researchers at Pennsylvania State University report in the latest issue of Science. Scientists have changed the colour of a dark-striped zebrafish to uniform gold by inserting a version of the pigment(色素) gene into a young fish. As with humans, zebrafish skin colour is determined by pigment cells, which contain melanosomes(黑色素). The number, size and darkness of melanosomes per pigment cell determines skin colour. It appears that, like the golden zebrafish, light-skinned Europeans also have a mutation(变异) in the gene for melanosome production. This results in less pigmented skin. However, Keith Cheng, leader of the research team, points out that the mutation is different in human and zebrafish genes. Humans acquired dark skin in Africa about 1.5 million years ago to protect bodies from ultra-violet rays of the sun(太阳光紫外线), which can cause skin cancer.But when modern humans leave Africa to live in northern latitudes, they need more sunlight on their skin to produce vitamin D. So the related gene changes, according to Cheng.Asians have the same version of the gene as Africans, so they probably acquired their light skin through the action of some other gene that affects skin colour, said Cheng. The new discovery could lead to medical treatments for skin cancer. It also could lead to research into ways to change skin colour without damaging it like chemical treatment did on Michael Jackson. The passage mainly tells us that ________.   A. people can not change their skin colour without any pain   B. the new discovery could lead to search into ways to change skin colour safely   C. pop king Michael Jackson often changed his skin colour as he liked   D. scientists have found out that people’s skin colour is determined by the gene It can be inferred from the passage that ________.   A. nowadays people who want to change their skin colour have to suffer a lot from the damage caused by the chemical  treatment                                 B. Europeans and Africans have the same gene   C. the new discovery could help to find medical treatments for skin cancer    D. there are two kinds of genes Scientists have done an experiment on a dark-striped zebra fish in order to ________.   A. find the different genes of humans’   B. prove the humans’ skin colour is determined by the pigment gene   C. find out the reason why the Africans’ skin colour is dark  D. find out the ways of changing peopl’s skin colour The reason why Europeans are light-skinned is probably that ________.   A. they are born light-skinned people   B. light-skinned Europeans have mutation in the gen for melanosome production   C. they have fewer activities outside          D. they pay much attention to protecting their skin The writer’s attitude towards the discovery is ________.   A. neutral                      B. negative                   C. positive                    D. indifferent