Triggers are indicated in each level's image by a brown circle
In case the circles are too hard to see, I also included little descriptions of their positions



I highly encourage trying to find the triggers' locations on your own. It didn't take much effort or time for me so I think you can do it too!

A-1: Top of the level
A-2: Top-right corner
A-3: Top-right corner
A-4: In right of the T-shaped corridor
A-6: To the left of the purple rectangle on the left
A-7: Inside of the middle umbrella's hole
A-8: Behind the purple block
A-9: Top-right of the spawn location

B-1: Through the corridor at the top of the level
B-2: On the left of the big lake
B-3: Behind a pillar on the right side of the level
B-4: On the right exit of the J-shaped hole
B-6: On top of the pillar on the left side of the level
B-7: On the left side of the left bomb
B-8: Hole between a lake and the left eye snake
B-9: Hole on top of the right eye snake

C-1: Hole underneath the spawn point
C-2: In front of the shattering glass
C-3: Through a gap that's opened after a lever is flicked
C-4: Bottom right of the level, underneath the clown box
C-6: Bottom left of the hole on the bottom right
C-7: Right underneath the top fire spinning box
C-8: Top gap of the P-shaped polygon with an eye snake
C-9: Bottom right corner of the level

D-1: On the left hole of the pepper shaker
D-2: Bottom right of the level, to the right of the big trigger-able block
D-3: On the top-left of the 2x1 water puddle
D-4: In the gap in the middle of the level
D-6: In the gap underneath the spawn point
D-7: In the hole to the right of the spawn point
D-8: On the top-right of the large water pool
D-9: In the dead-end underneath the three moving plate tiles

E-1: Gap underneath the top white eye
E-2: Gap between two blocks on the right side of the level
E-3: Underneath the lever to the right of the spawn point
E-4: On the left gap of the tile the left snake crawls on
E-6: In the hole of the right-most red bubble
E-7: On top of the cannon underneath the right white eye
E-8: In the gap at the middle of the level
E-9: Behind the exit (be extra careful to not go through the exit accidentally!)