Normally tree rings will disappear, damage, or go missing when the tree doesn't have enough nutrients, another factor that contributes to that is drought. 
        
             
        
        
        
Answer: 
The correct answer is option (D) there was a time in Pennsylvania history when dinosaur existed. 
Explanation: A paleontologist discovers a rock slab on which the footprints of a Pennsylvania dinosaur are well preserved. This means that there was a time in Pennsylvania history when dinosaur existed in this area. Here fossil is a fragment of rock on which the footprints of dinosaur is present. 
 
        
             
        
        
        
In geology, a graded bed is one characterized by a systematic change in grain or clast size from one side of the bed to the other. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into progressively finer onesI just learned about this in our rocks and minerals unit for science, 
 
        
             
        
        
        
Answer:
1. Presynaptic cell 2. Postsynaptic cell and 3. Neurotransmitters 
Explanation: Just did the assignment