体内许多酶的活性与其分子中半胱氨酸残基上的巯基直接有关，有些毒物能与酶分子的巯基结合

（）是指无污染的安全、优质、营养食品，生张原料或产品在生产过程中不用或少用化学肥料、化学农药和其他化学物质。

后天之本是（）

A.心

B.肝

C.脾

D.肺

E.肾

桶形畸变是（）镜头产生的。

下面关于菜单的叙述中错误的是【 】

A．各级菜单中的所有菜单项的名称必须唯一

B．同一子菜单中的菜单项名称必须唯一,但不同子菜单中的菜单项名称可以相同

C．弹出式菜单用Popup Menu方法弹出

D．弹出式菜单也用菜单编辑器编辑

Remember Second Life, the virtual world that was supposed to become almost as important as the first one Now populated by no more than 84,000 avatars at a time, it has turned out to be a prime example of how short-lived Internet fads can be. Yet if many adults seem to have given up on virtual worlds, those that cater to children and teenagers are thriving. Several have even found a way to make money.

In America, nearly 10 million children and teenagers visit virtual worlds regularly, estimates eMarketer, a market researcher-a number the firm expects to increase to 15 million by 2013．As in January, there were 112 virtual worlds designed for under-18s with another 81 in development, according to Engage Digital Media, a market research firm.

All cater to different age groups and tastes. In Club Penguin, the market leader, which was bought by Disney in 2007 for a whopping $ 700 million, primary-school children can take on a penguin persona, fit out their own igloo and play games. Habbo Hotel, a service run from Finland, is a global hangout for teenagers who want to customise their own rooms and meet in public places to attend events. Gala Online, based in Silicon Valley, offers similar activities, but is visited mostly by older teens who are into Manga comics.

Not a hit with advertisers, these online worlds earn most of their money from the sale of virtual goods, such as items to spruce up an avatar or a private room. They are paid for in a private currency, which members earn by participating in various activities, trading items or buying them with real dollars.

This sort of stealth tax seems to work. At Gala Online, users spend more than $1 million per month on virtual items, says Craig Sherman, the firm’s chief executive. Running such a virtual economy is not easy, which is why Gaia has hired a full-time economist to grapple with problems that are well known in the real world, such as inflation and an unequal distribution of wealth.

There are other barriers that could limit the growth of virtual worlds for the young, but the main one is parents. Many do not want their offspring roaming virtual worlds, either because they are too commercial or are thought to be too dangerous. Keeping them safe is one of the biggest running costs, because their sponsors have to employ real people to police their realms.

Youngsters are also a fickle bunch, says Simon Levene of Accel Partners, a venture- capital firm. Just as children move from one toy to another, they readily switch worlds or social networks, often without saying goodbye.

Even so, Debra Aho Williamson, an analyst at eMarketer, believes "these worlds are a training ground for the three-dimensional web". If virtual worlds for adults, which so far have been able to retain only hardcore users, manage to hang on for a few years, they may yet have a second life.

What may "stealth tax" in "This sort of stealth tax seems to work" refer to（）

A. Online world promotes transaction without seeing the currencies, thus boosting the consumption

B. Things in the online world do not need to be taxed, and then is cheaper than actual ones

C. Companies have already paid the tax for the players

D. People buy things in the online world in a largely unnoticed way, either by himself or by others