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下列给出一棵二叉树，按照后序法周游二叉树，结点的输出序列是
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An ad on the back cover of a magazine is usually not an effective advertisement.
Section A
Directions: In this section, there are 10 incomplete sentences. You are required to complete each one by deciding on the most appropriate word or words from the 4 choices marked A , B, C, and D.
There was nothing they could do ______.
拍卖人不得以竞买人身份参加本拍卖机构举办的拍卖活动，但可以委托他人代为竞买。
Apparently, overuse of salt causes high blood pressure and hypertension, the cause of half the deaths in the United States every year. A few years ago the anti-salt campaigners raised such an uproar that salt was banned from baby food.Currently pressure is being applied to food manufacturers to oblige them to label their products to show sodium content. Because doing so would cost manufacturers money, they argue that they have no idea how much salt remains on such things as potato chips and how much sticks to the bag. Furthermore, salt isn't the only harmful ingredient in food.If the manufacturer has to provide sodium content, why not require him to list every ingredient and specify which are harmful to our health？ Cigarettes have a warning printed on them. Shouldn't the same type of warning appear on canned foods that are notoriously over- salted？
There are endless ifs and buts in the controversy, but the most telling of these is the questionable proof of salt's effect upon the blood pressure. True, people who cut their salt intake lowered their blood pressure, but where is the scientific proof that something other than salt didn't do the trick？ The most common means of providing dubious proof that salt causes hypertension is to compare societies that use little salt with those that use mountains of salt in their daily diets. Which group has the higher rate of hypertension？ Whose blood pressure is lower？ What happens when salt is introduced into a group where salt is a novelty？ Does the blood pressure rise significantly？ Studies of the Japanese indicate that as the world's greatest salters, they suffer the most from hypertension. On the other hand, the simple, salt-free cooking of several tribes in the Solomon Islands has kept older tribesmen and women from developing hypertension and high blood pressure, ailments traditionally killing their peers in America.No account is taken of the effects of inflation, recession, pollution, crime, and sundry (多种多样的) other ills to which Americans, unlike people on primitive islands, are exposed.
To salt or not to salt？ That is the question. Now that the question has arisen, it must not be treated with levity(轻率) but, rather, with searching scientific investigation so that those of us who are preoccupied with both savory(薄荷) food and longevity may decide which of the two is worth its salt.
The attitude of the author of this passage toward the salt controversy is that______。
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