He had a strange hairstyle and he instantly became the ______ of attention the moment he stepped into the classroom. A. aim B. focus C. direction D. point 答案:B如图,已知AD∥BC,∠1=∠2,说明∠3+∠4=180°,请完成说明过程,并在括号内填上相应依据: 解:∠3+∠4=180°,理由如下: ∵AD∥BC(已知), ∴∠1=∠3( ) ∵∠1=∠2(已知) ∴∠2=∠3(等量代换); ∴ ∥ ( ) ∴∠3+∠4=180°( )