估算，我最棒。
787×7≈ 303×19≈ 5605÷8≈ 6933÷7≈
5600；6000；700；1000
细菌和真菌有的需要氧属________型；有的属________型，如乳酸菌。
He said that he would write to us but so far we ___________ from him.
A．didn’t hear
B．haven’t heard
C．shouldn’t have heard
D．hadn’t heard
