小熊商店运进3箱方便面，每箱24包．卖出19包后，还剩多少包？
3×24-19
=72-19
=53（包）
答：还剩53包．
鉴别Na2CO3与NaHCO3两种无色溶液的最好方法是（ ）
A．加稀硫酸
B．加热
C．加CaCl2溶液
D．加Ca（OH）2溶液
Everybody in the little town knew my first teacher, so we had no ____ finding his home. [ ]
A. difficulties in
B. difficulty with
C. difficulty
D. difficulty to
