In this task, you are given a question and a context passage. You have to answer the question based on the given passage.

What Apollo flight followed the one in 1967?, Context: The delay in the CSM caused by the fire enabled NASA to catch up on man-rating the LM and Saturn V. Apollo 4 (AS-501) was the first unmanned flight of the Saturn V, carrying a Block I CSM on November 9, 1967. The capability of the Command Module's heat shield to survive a trans-lunar reentry was demonstrated by using the Service Module engine to ram it into the atmosphere at higher than the usual Earth-orbital reentry speed. This was followed on April 4, 1968, by Apollo 6 (AS-502) which carried a CSM and a LM Test Article as ballast. The intent of this mission was to achieve trans-lunar injection, followed closely by a simulated direct-return abort, using the Service Module engine to achieve another high-speed reentry. The Saturn V experienced pogo oscillation, a problem caused by non-steady engine combustion, which damaged fuel lines in the second and third stages. Two S-II engines shut down prematurely, but the remaining engines were able to compensate. The damage to the third stage engine was more severe, preventing it from restarting for trans-lunar injection. Mission controllers were able to use the Service Module engine to essentially repeat the flight profile of Apollo 4. Based on the good performance of Apollo 6 and identification of satisfactory fixes to the Apollo 6 problems, NASA declared the Saturn V ready to fly men, cancelling a third unmanned test.
6