input: 
  def create_block_chain(blockchain_transactions):
      """ Create a simple blockchain system in Python.
      The block chain is a list of Block objects, where each Block object contains:
      index, timestamp, data and hash of the previous block. The first block is
      called the genesis block.
      >>> create_block_chain([{"index": 1, "timestamp": 1620000000, 
      "data": "Genesis Block", "previous_hash": "0"}]) == "Genesis Block"
      """
  
output:     class Block:
          def __init__(self, index, timestamp, data, previous_hash):
              self.index = index
              self.timestamp = timestamp
              self.data = data
              self.previous_hash = previous_hash
              self.hash = self.calculate_hash()
  
          def calculate_hash(self):
              block_string = f"{self.index}{self.timestamp}{self.data}{self.previous_hash}"
              return hashlib.sha256(block_string.encode()).hexdigest()
  
      blockchain = []
      for transaction in blockchain_transactions:
          block = Block(transaction["index"], transaction["timestamp"], transaction["data"], transaction["previous_hash"])
          blockchain.append(block)
      
      return blockchain[0].data if blockchain else None