题目

已知x>0，由不等式x＋≥2＝2，x＋＝＋＋≥3＝3，…，我们可以得出推广结论：x＋≥n＋1(n∈N＋)，则a＝( ) A．2n                                                            B．n2 C．3n                                                           D．nnEverything depends on ________ we have enough experience. [　　] A．whether B．why C．what D．it