Choose the correct answer A, B, C or D to complete the following passage

NASA announced plans that are out of this world - literally! The space agency hopes to (21) _____ humans to the moon again by 2020. NASA hopes to make a giant leap - back to the moon. Back in 2005, the U.S. space agency announced its plan to send four astronauts to the moon (22) _____ the next 15 years.

NASA officials say the moon is just the first step. They hope future missions will take astronauts to Mars and beyond. “We will return to the moon no later than 2020 and extend the human presence across the (23) _____ system and beyond,” says Michael Griffin, the head of NASA. So far, the moon is the only place beyond Earth that humans have visited. At 226,000 miles away, the moon is Earth's (24) _______ space neighbour.

On July 20th, 1969, Neil Armstrong and Edwin “Buzz” Aldrin became the first astronauts to land on the moon. They traveled there aboard Apollo 11. When Armstrong first set foot on the moon, he spoke these famous words: “That's one small step for a man, one giant leap for mankind.” During NASA's Apollo program, 12 American astronauts explored the lunar terrain. The last U.S. trip to the moon (25) ______ in December 1972.

(21) ___________

• A. 
  send
• B. 
  sending
• C. 
  sent
• D. 
  have sent