Read and choose.(给下面的图片选择合适的描述。)   

1. He is at school now. He will be home at four o'clock.    

2. Father is at the office now. He will be home at six o'clock.    

3. Maomao is on the train. She will be home at nine o'clock.

4. Lily is in the park now. She will be home at seven o'clock.

函数fun的返回值是（ ）。　　fun(char*a，char*b)　　{　　　　intnum=0，n=0；　　　　while(*(a+num)!=''\0'')num++；　　　　while(b[n]){*(a+num)=b[n]；num++；n++；}　　　　returnnum；　　}

A．字符串a的长度

B．字符串b的长度

C．字符串a和b的长度之差

D．字符串a和b的长度之和