单句语法填空
(1) (2018·全国Ⅰ)The review says it is more effective at lengthening life __________ walking, cycling or swimming.
(2)(2016·全国Ⅲ)Over time, __________ the population grew, people began cutting food into small pieces so it would cook more quickly.
(3)(2018·全国Ⅱ)Corn uses less water __________ rice and creates less fertilizer(化肥) runoff.
(4)(2016·四川)It was time for her to have a new baby, __________ it was also time for the young panda to be independent.