Brilliant game, and phenomenal work! Having to control which blocks were active and which weren't was a creative mechanic and it was implemented really well, and each level had a different unique challenge to it as well, which was great. The movement felt good too, and the little amounts of squash and stretch and rotation really added to that player feedback. One thing I would say though is that jumping felt a bit off; I kept thinking I could jump two blocks when I could only reach one. Maybe it was just me, but decreasing the jump height a little could've been useful there. The mechanic of switching between player and radiation was nice too, although it did get a little tedious at times - having to jump, move the line across the screen, land, move the line to the other side, repeat - especially when you failed a jump or managed to jump past the puzzle piece somehow, like me. Being able to use different control sets was nice though, since it was more comfortable for me to use IJKL / Z, and I didn't accidentally turn on sticky keys. There were a couple of bugs I noticed too, but nothing game-breaking or major, so that was all fine, especially considering it was made in three days.
And as for the artwork, I found it wonderful! It was a very simple style, but it still managed to look great. I did really like the environment too - it was abstract and definitely not complex, but it didn't feel empty either, which is great. The colours stood out as nice and vibrant, especially with the contrast in the background, but not oversaturated or too bright. The way the UI matched the style of the game itself was a nice touch too and I loved the little story pictures between levels - helped add a lot of charm and flavour to the game. I loved the atmosphere and the sound too - some nice ambient background noise was nice, and the variation in sound effects kept them fresh.
With that all being said, I think it was an excellent game! I'd love to hear about anything that happens with this game in future - it's a fun little puzzle game that's well polished and well implemented, so phenomenal work!