var arr = new Array(new Array(9,0,3,4,5) , ['a' , 'b' , 'c'] , new Array(2,9,0,6)); 則arr[1][3]=()
A.c B.6 C.4 D.undefined
A.NaN!=null B.NaN==NaN C.{}!=false D.null!=false
A.string B.length C.非法字符 D.this