per my understanding i could split this into two sentences.
1) In 1839, experiments with hydrogen and water performed by Sir William Grove led to the development of fuel cells (Sub- experimnets, verb- led )
2) Though the use of these cells was originally considered overly costly, today the fuel cell, which can efficiently power everything from large generating plants and NASA rockets to small portable communication devices, seems to have become a popular and promising source of power.
This sentence has two clauses. One independent & other dependent.
Dependent Clause - Though the use of these cells was originally considered overly costly (Sub - the use, Verb - was considered)
Independent clause - today the fuel cell, which can efficiently power everything from large generating plants and NASA rockets to small portable communication devices, seems to have become a popular and promising source of power.
(Sub - the fuel cell , Verb - seems to have, modifier - which can ......communication devices)
Per this, Sub - fuel cell is singular which requires a singular verb .
But the correct answer has plural verb.
Could anyone please tell me where I am going wrong?