Why we use let's while talking?

Read sentence 13 from the draft. some plants retain a great deal of their foliage through much of the spring and summer, but the leaves do not retain their green color for long. which detail would be best to add after sentence 13 to develop the topic? a. since the green color of the leaves that are bright and abundant during the warmer months will disappear by the time it is fall, it is important that the leaves are trimmed and kept freshly cut so that they will last for as long as possible. b. while the temperature outside may dictate when the fall colors of leaves appear, it is only one of many environmental factors that play a part in the changing of colors from green leaves to fall-colored leaves during the cooler months. c. if people want to enjoy the green color of leaves longer, trees should be planted where the environment is warmer and the days longer because chlorophyll in the leaves can only be manufactured and replaced with sunlight and warmth. d. the green chlorophyll normally masks the fall-colored pigments during the warmer months, but as sunlight becomes less abundant and trees become frozen, the fall colors of leaves become more visible as chlorophyll disappears.

How do i correct this sentence correctly?

Why didn't van gogh knock on breton's door? it seemed imposing and a bit scary. it looked alarmingly middle-class. he couldn't find the door. he saw a book that he liked instead.