The diagram below shows the radioactive decay of cesium (135Cs). What is the half-life of 135Cs? 
Definitions:
Groin Vault
An architectural element formed by the intersection at right angles of two barrel vaults, creating a vaulted ceiling with a distinctive ridge along the intersection.
Tribune
In church architecture, a gallery over the inner aisle flanking the nave.
Speyer Cathedral
A Romanesque cathedral located in Speyer, Germany, known for its monumental architecture and historical significance as the final resting place of several Holy Roman Emperors.
Modern Country
A nation-state characterized by a developed economy, technological infrastructure, and a contemporary social structure.
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