You left out a lot. What prompted Nicodemus to ask Jesus "How can these things be?" was Jesus saying 3-8 Jesus answered him, (and here N had not even asked Jesus anything, he had only made a statement about Jesus) "Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God." Nicodemus said to him, "How can a man be born when he is old? Can he enter a second time into his mother's womb and be born?" Jesus answered, "Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God. That which is born of the flesh is flesh, and that which is born of the Spirit is spirit. Do not marvel that I said to you, 'You must be born again.' The wind blows where it wishes, and you hear its sounds, but you do not know where it comes from or where it goes. So it is with everyone who is born of the Spirit."
Jesus is talking about being born again. That it is something the Spirit does and no one can see the kingdom of heaven unless he is first born again. Nicodemus wants to know how this born again "thing" can be. He asks, "How can this be." Jesus answered him, "Are you the teacher of Israel and yet do not understand these things?
Why did Jesus rebuke him for not knowing already what he had just said since he was a teacher of the Law? Linking the "water" and the "Spirit" as he did are likely references to OT passages of God pouring out his Spirit in the end times (the post resurrection days) (Isa 32:15; 44:3; Ez 36:25-27).
11-15 Trutly, truly, I say to you, we speak of what we know, and bear witness to what we have seend, but you do not receive our testimony. If I told you earthly things and you do not believe, how can you believe if I tell you heavenly things? No one has ascended into heaven except he who descended from heaven, the Son of Man. And as Moses lifted up the serpent in the wilderness, so must the Son of Man be lifted up, that whoever believes in him may have eternal life.
Here Jesus is making another comparison to himself and the prefiguration of the snake on a pole. He is telling Nicodemus who he is. But he does not say an option is being offered as in the OT. He does not say look upon him and receive eternal life. He simply says whoever believes in him will have eternal life.