求助OG18 SC680的agreement问题

曰生白告

原题：
Unlike the virginal, whose single set of strings runs parallel to the front edge of the instrument, the harpsichord’s several sets of strings are placed at right angles to its front edge.

(A)whose single set of strings runs parallel to the front edge of the instrument, the harpsichord’s several sets of strings are
(B)with a single set of strings running parallel to the front edge of the instrument, the several sets of strings of the harpsichord are
(C)which has a single set of strings that runs parallel to the front edge of the instrument, in the case of the harpsichord, several sets of strings are
(D)which has a single set of strings that run parallel to the front edge of the instrument, the harpsichord has several sets of strings
(E)in which a single set of strings run parallel to the front edge of the instrument, the harpsichord’s several sets of strings are
答案是选D 但是D里which has a single set of strings that run...里run为何不是runs？ a set of strings被视为复数吗？但是也有反例啊：
A clause is a set of words that contains a subject and a working verb.
网上搜了一下说都有可能 那怎么判断和区分呢 求NN指点

LLEOE

that引导的定语从句修饰strings而不是修饰a single set, strings在定语从句中作主语，是strings run parallel to the front edge of the instrument, 而不是a single set runs parallel to the front edge of the instrument