脐带过短是指脐带短于()
A.20cm
B.30cm
C.40cm
D.50cm
E.55cm
参考答案:B
I made a promise to myself ________ this year,my first year in high school,would be different.(2012·浙江)
A.whether
B.what
C.that
D.how
常见工程塑料有(),(),(),其中自润滑性能良好适合作轴承齿轮的有尼龙,价格低廉,用途非常广泛的是聚氯乙烯,化学稳定性极好,有塑料王之称的是聚四氟乙烯.