He apologized ________ late.
As we know, too much ________ to Xrays can cause skin burns, cancer or other damage to the body.
The lesson Mr Li gave just now was too difficult for the class to ________.
The scientists recommend that studies ________ to find out what side effects the medicine might cause.
An advertisement is useful in ________ it lets customers know of the kinds of goods in the shops.
—I need some fresh air, so I am going out for a walk. —________.