Answer:
Definitely C, but also B (I do not recall anything about immigrants constructing the Hoover Dam)
Step-by-step explanation:
From 1863 and 1869, roughly 15,000 Chinese workers helped build the transcontinental railroad.
Unable to attract workers from Jamaica, officials recruited in Barbados and by 1905 more than 22,000 Barbadians were working on the canal.
Spaniards and Italians led the pack of European immigrants who worked on building the Panama Canal 100 years ago.
Because those two definitely had immigrant assistance, i would presume that it is all of the above, but I am not 100% sure about the dam.