蛋白质的回收浓缩通常选用
A、反渗透
B、超滤
C、微滤
D、电渗析

单项选择题During the 1860's, canned food products were ______.A.unavailable in rural areasB.shipped in refrigerator carsC.available in limited quantitiesD.a staple part of the American diet

单项选择题The author implies that in the 1920's and 1930's home deliveries of ice ______.A.decreased in numberB.were on an irregular scheduleC.increased in cost .D.occurred only in the summer