41. Excavators at the Indus Valley site of Harappa in eastern Pakistan say the discovery of inscribed shards dating to circa 2800 - 2600 B.C. indicate their development of a Harappan writing system, the use of inscribed seals impressed into clay for marking ownership, and the standardization of weights for trade or taxation occured many decades, if not centuries, earlier than was previously beliieved.

a) indicate their development of a Harappan writing system, the use of
b) indicate the the development of a Harappan writing system, using
c) indicates that their delevelopment of a Harappan writing system, using
d) indicates the development of a Harappan writing system, their use of
e) indicates that the development of a Harappan writing system, the use of

.. My question is how do you know which noun goes with "indicate". I assumed that the noun was EXCAVATORS and because that is plural, i assumed "indicate" plural as well. Please explain.