正确答案：
答案解析：
Wemust______thattheexperimentiscontrolledasrigidlyaspossible.
Aftersomanydayswithoutrain,thegroundquickly________thelittlerainthatfelllastnight.
Thewealthofacountryshouldbemeasured________thehealthandhappinessofitspeopleaswellasthematerialgoodsitcanproduce.