subject-verb agreement

你要求解釋看來你還未明白英語文法結構時這種數量組的動詞不是按總量計而是按組量計的。

a pair of socks的數量在實物上是兩隻但按組合上是一個單位，所以是看成單數的。
two pairs of...的單位是pair(雙/對)，而有兩個單位的組合，所以看成是眾數。a bunch/bundle(一扎/一包) of...東西=單數
bunches/bundles of...東西=眾數

a school of fish [fish是眾數意思] = 單數(一羣魚)
schools of fish = 眾數(多羣魚)

a herd of "animals" = 單數 (一羣動物)
herds of "animals" = 眾數 (多羣動物)

推至a body (group) of volunteers (people/men/women/children,etc) = 一(組)羣的人 = 單數。has是對的。

Collective nouns are singular when we think of them as groups and plural when we think of the individuals acting within the whole (which happens sometimes, but not often).

e.g. A pair of soldiers were standing guard at the entrance.

A pair of socks is …
Two pairs of socks are …
A bunch of flowers is ….
Two bunches of flowers are ….
A body of volunteers has ….

Pair, bunch and body are collective nouns.

1. a pair of socks 先正確, 本身你都錯了, a piece of sock 先係一隻袜用 is
無論 a pair / two pairs 都係用are

a bunch of flowers 用 are, 因為 flower 係 countable (可數)
a bunch 一束點都多過一支
所以 X bunches of flowers (記得flower 要加 s) 都係用 are

2. 同樣道理, volunteers 係人, a body of 即係一組/ 多過一個人,
自然要用 have