Making triangles

This task is about patterns and rules.

Fill in the spaces below to show the next 2 triangular numbers in the sequence. Draw a diagram for these numbers.

3 is a triangular number: 

6 is the next triangular number: 

a)
__________ is the next triangular number. Diagram:
 
 
 
 
b)
__________ is the next triangular number. Diagram:
 
 
 
 
c)
Complete this sentence: "Each row is n + _____ of the row before it."
 
d)
In a supermarket Jane builds up a display of tins of soup like this:
patterns of stacked cans
If there were a total of 28 tins, how many tins high would the stack be?
 
(A) 6
(B) 7
(C) 14
(D) 28
(E) 56