In her backyard garden, Gabriella has 12 tomato plants in a row. As she walks along the row, she notices that each plant in the row has one more tomato than the plant before. If she counts 186 tomatoes in total, how many tomatoes are there on the last plant in the row?
(A) 15
(B) 16
(C) 20
(D) 21
(E) 22
Suppose that Gabriella's twelve plants had 1,2,3, 4, 5, 6, 7, 8, 9, 10, 11, and 12 tomatoes. How many tomatoes would she have in total? Adding these numbers up, she would have 78 tomatoes.

But we know that she has 186 tomatoes, so there are 108 tomatoes unaccounted for.

Since the number of tomatoes on her plants are twelve consecutive whole numbers, then her plants must each have the same number of extra tomatoes more than our initial assumption. How many extra tomatoes should each plant have? 108 tomatoes spread over 12 plants gives 9 extra tomatoes each. Therefore, the plants have 10,11, 12, 13,14, 15, 16, 17, 18, 19, 20, and 21 tomatoes, and the last one has 21 tomatoes. (We can check that there are indeed 186 tomatoes by adding $10+11+\ldots+21$.)

ANSWER: (D)