21. Attention, usually referred to as the ________ of processing resources, is the cognitive process of selectively concentrating on one aspect of the environment while ignoring other things.
(A) application
(B) allocation
(C) classification
(D) integration

(A) application
an official request for something, usually in writing
（通常指書面的）申請，請求
Free information will be sent out on application to (= if you ask) the central office.
I've sent off applications for four different jobs.
Have you filled in the application form for your passport yet?
Argentina has submitted an application to host the World Cup.
a computer program that is designed for a particular purpose
應用軟體
spreadsheet applications
a way in which something can be used for a particular purpose
用途，用法；應用
The design has many applications.
the determination to work hard over a period of time in order to succeed at something
勤奮；努力；專心致志
Joshua clearly has ability in this subject but lacks application.
the act of spreading or rubbing a substance such as cream or paint on a surface, or a layer of cream or paint
塗抹；敷用
Leave the paint to dry between applications.
Regular application of the cream should reduce swelling within 24 hours.
a way in which a rule or law, etc. relates to or is important for someone or something
（法律，規定等的）適用
The new laws have (a) particular application to the self-employed.

(B) allocation
the process of giving someone their part of a total amount of something to use in a particular way
分配；分派；撥給，劃撥
Allocation of the grants was handled by a charity that provides support and advocacy for disabled children.
the part of a total amount of something that someone is given to use in a particular way
分配物；劃撥款項
The university is offering moving allocations to anyone who is being displaced from properties it acquires.
Higher budgetary allocations for education mean more teachers can be hired and classes can be smaller.

(C) classification
the act or process of dividing things into groups according to their type
將…分類，將…歸類；把…分級
Do you understand the system of classification used in ornithology?

(D) integration
the action or process of successfully joining or mixing with a different group of people
融合
To promote integration, several schools' catchment areas were merged.
the action or process of combining two or more things in an effective way
整合
He creates a seamless integration of contemporary and historic images.
The heads of government were trying to encourage regional integration.

資料來源：劍橋辭典