Read this: Greeks in pre-Roman Gaul were mostly limited to the Mediterranean coast of Provence. The first Greek colony in the region was Massalia, which became one of the largest trading ports of Mediterranean by the 4th century BCE with 6,000 inhabitants. Massalia was also the local hegemon, controlling various coastal Greek cities like Nice and Agde. The coins minted in Massalia have been found in all parts of Ligurian-Celtic Gaul. Celtic coinage was influenced by Greek designs, and Greek letters can be found on various Celtic coins, especially those of Southern France. Traders from Massalia ventured inland deep into France on the Rivers Durance and Rhône, and established overland trade routes deep into Gaul, and to Switzerland and Burgundy. The Hellenistic period saw the Greek alphabet spread into southern Gaul from Massalia (3rd and 2nd centuries BCE) and according to Strabo, Massalia was also a center of education, where Celts went to learn Greek. A staunch ally of Rome, Massalia retained its independence until it sided with Pompey in 49 BCE and was then taken by Caesar's forces.

Massalia was the center of education according to whom?
What is the answer? (If it cannot be answered, return "unanswerable")
Strabo