The questions can be confusing for students of British English:
http://en.wikipedia.org/wiki/American_a ... ifferences
Singular and plural for nouns
In BrE, singular nouns that describe multiple people are often treated as plural, particularly where one is concerned with the people constituting the team, rather than with the team as an entity. The singular form is usually used in American. For example, British "the team are worried"; American "the team is worried". Americans may use the plural form when the individual membership is clear, for example, "the team take their seats" (not "the team takes its seat(s)"), although it is often rephrased to avoid the singular/plural decision, as in "the team members take their seats". The difference occurs for all collective nouns, both general terms such as team and company and proper nouns (for example, where a place name is used to refer to a sports team). Proper nouns which are plural in form take a plural verb in both AmE and BrE. Examples:
BrE: "The Clash are a well-known band." AmE: "The Clash is a well-known band." Both: "The Beatles are a well-known band."
BrE: "Pittsburgh are the champions." AmE: "Pittsburgh is the champion." Both: "The Steelers are the champions".
Use of the singular verb is not wrong in such instances in BrE. At least one authority (E. Gowers, The Complete Plain Words, 1986) indicates that either is acceptable (provided that usage is not mixed or inconsistent within the same document), and that (as implied above) the choice of verb form may be chosen according to whether the emphasis is on the body as a whole or on the individual members (for example, "A committee was appointed ...; but "the committee were unable to agree ...").
so "half" and "group" should be treated as singular on GMAT ion your examples?
"Amicus Plato, sed magic amica veritas"