典型性先天性髋脱位的主要发病因素为（）.

A.髋臼发育不良

B.股骨颈前倾角增大

C.关节囊、韧带松弛

D.股骨头发育不良

E.股骨颈短

现有一个文件file21.txt,其内容是: abCdEf, 执行下列程序之后，输出的结果是______。package ch1;import java,io.*;public class ex21{ static String name = "ch1\\file21.txt"; public static void main(String[] args) { try { readFile (); } catch(IOException ioe) { System.out.println(ioe.getMessage()); } } static void readFile () throws IOException { BufferedReader br = null; try { File f = new File(name); FileReader fr = new FileReader(f); br = new BufferedReader(fr); String str= br.readLine(); System.out.println(str.toLowerCase()); } finally if(br != null) br.close (); } }}

A．AbCdEf

B．abcdef

C．aBcDeF

D．ABCDEF