如果自行车链条的长度比标准长度长2mm,记作+2mm,那么比标准长度短1.5mm,可记作( )。
-1.5mm
My brother often __________ my mum ________ the house on Sunday.[ ]
A. help; cleans
B. help; clean
C. helps; clean
D. helps; cleans
婴儿添加辅助食品时,一种辅食一般应经过()天的适应期。
A.2~3
B.3~5
C.5~7
D.7~9
E.9~11