— The _____ of July is your birthday. What do you usually do?
— I usually help my mother with some housework because I want her to have a rest _____ that day.
A：first; on
B：one; on
C：first; in
D：one; in
下列地形中受人类活动影响最大的是（ ）
一合金块体积为3×10﹣3米3，浸没在水中时，求合金块受到的浮力F浮．
下列哪组可能是原始大气的成分（ ）
