Ivan is an antiques dealer. He imported two extremely rare statues, created by the same sculptor, and placed an advertisement in a specialist magazine offering them for sale at a price of 1 million roubles per statue.Ekaterina replied to the advertisement immediately by post, agreeing to pay 1 million roubles to purchase one statue. However, she then had doubts about whether the statue would be a good bargain and sent a fax on the same day that her letter was despatched to say that she had changed her mind.On the same day, Leonid replied to the advertisement by post, confi rming that he would be prepared to pay 800.000 roubles for one statue.Having been away on a business trip for a few days, Ivan arrived at his offi ce and read the letter and fax sent by Ekaterina and the letter sent by Leonid. He sent faxes to both customers confi rming that he would supply the statues immediately. In the fax to Ekaterina he informed her that her fax could not cancel the deal as she had accepted his offer.On the same morning, Nikolai visited Ivan and offered to purchase both statues for 1.5 million roubles each. Ivan accepted this proposal and sold the statues immediately.Required:Advise Ekaterina, Leonid and Nikolai whether or not they have entered into legally binding contracts. (10 marks)

与呼吸系防御有关的结构有（）。

A.鼻毛

B.纤毛

C.混合腺

D.肺巨噬细胞

E.淋巴组织