Before scientists learned how to make a synthetic growth hormone, removing it painstakingly in small amounts from the pituitary glands of human cadavers.

• Ascientists learned how to make a synthetic growth hormone, removing it painstakingly  
• Bscientists had learned about making a synthetic growth hormone, they had to remove it painstakingly  
• Cscientists learned how to synthesize the growth hormone, it had to be painstakingly removed  
• Dlearning how to make a synthetic growth hormone, scientists had to remove it painstakingly  
• Elearning how to synthesize the growth hormone, it had to be painstakingly removed by scientists