6个千、6个百和6个一组成的数是______位数，写作______．
6个千、6个百和6个一组成的数是6606，是四位数；
故答案为：四，6606．
Helen has got ________ it takes to be a great tennis player. In other words, she has the potential to become world famous.
A．how
B．what
C．when
D．that
引起小儿脊柱后凸最常见的原因是（）
A.结核
B.肿瘤
C.外伤
D.佝偻病
E.强直性脊柱炎
