Most consumers are familiar with bar codes. Those black-and-white stripes appear on almost

every product purchased. Often the codes are scanned at the checkout counter, causing the item name and its price to appear on (141) the cash register screen and the receipt. This gives a record to the consumer and helps store management monitor inventory.

Many delivery services use bar codes to track the progress of each envelope, package, or crate in transit. Every item (142) a bar code, which is read at a number of points from the moment it leaves the sender's hands to the moment the recipient accepts the delivery.

An interesting use for the bar code has been developed by a scientist who works with bees. The scientist attached bar codes to the backs of individual bees in order to (143) them and to follow their movements. Laser scanners at the entrance of the hive read the bar codes to monitor how long each bee had been gone from the hive. By keeping (144) of individual bees, scientists hope to learn more about honey production. Knowing how far bees travel to gather pollen and how many trips they make back to the hive could eventually help beekeepers predict honey yields.

(41)

A．so

B．either

C．each

D．both