以提供某作物为回报而与他人换取所需产品的行为构成（）
A交换
B交易
C买卖
D洽谈
参考答案：A
He is not ________to do his homework., which makes his parent worried
A．careful enough
B．enough careful
C．carefully enough
D．enough carefully
影响混凝土工作性的因素？
