题目

f （x）=﹣sin（x+ ） sin（x﹣ ）的最小正周期和一条对称轴方程为（   ） A . 2π；x=kπ+ ，k∈Z B . 2π；x=kπ+ ，k∈Z C . π；x= kπ+ ，k∈Z D . π；x= kπ+ ，k∈Z 答案：CWhatever we may need to buy，we can use our credit cards．This is a change from the older days when we had to first check whether cards were accepted．Today，everybody, from retailers（零售商) to lenders，accepts credit cards．Attitudes towards credit cards have also changed a lot．People used to be very careful of using their cards；but this is no longer the case. People today prefer to take out their cards as soon as they get to the cash counter．A swipe (刷卡) and a sign are all it takes，and you can walk home with the bill and all the things you buy．    The credit card is much easier to carry than cash and it is not at all surprising that the former becomes the most preferred way of payment．A credit card takes up very little space in our wallets，and yet，it holds the power of a large sum of cash．Paying by credit cards is also a good way of keeping bills on where all of our money goes．      Moreover，it has become much easier to get credit cards these days．I constantly have cold callers calling up to offer me “great new credit card deals”．Salesmen occasionally walk into our office offering discounts if a group of people decide to sign up a certain kind of credit card plan at the same time．However，this can also cause a problem．Nowadays，we always run the risk of getting some kinds of wastes．When the terms seem too good to be true，we should make sure that we always read the printed items carefully．Before signing for a credit card，it is necessary to know its advantages and disadvantages．The only way to deal with this is to do our homework, before we go credit card hunting．    1.We learn from Paragraph 1 that      ．    A．people can use credit cards anytime，anywhere    B．people used to have to check up on credit cards    C．people are more careful using credit cards nowadays   D．people can take their goods home just by swiping their cards  2.One of the main advantages of credit cards is that    ．  A．they occupy less space compared with other modes of payment   B．they are more powerful than cash     C．they are easy to get，and save money     D．they help users keep track of what they spend on   3.When signing for a credit card，people should     .  A．ask for a discount    B．understand the dangers     C．read every word of the contract carefully    D．understand that the salesman is most likely to offer false information    4.In the last sentence the writer tells us that      ．   A．we should know the advantages and disadvantages of credit cards before using them   B．we should finish work before going shopping with a credit card    C．we should check which credit cards are accepted before using them  D．we should make sure that our credit cards are real before using them