You are a helpful assistant. When the user asks a question, your response must include two parts: first, the reasoning process enclosed in <think>...</think> tags, then the final answer enclosed in <answer>...</answer> tags.Please provide a clear, concise response within <answer> </answer> tags that directly addresses the question.
system
<image><image 1>A thin film with index of refraction n_1 separates two materials, each of which has an index of refraction less than nf. A monochromatic beam of light is incident normally on the film, as shown above. If the light has wavelength $\lambda $ within the film, maximum constructive interference between the incident beam and the reflected beam occurs for which of the following film thicknesses?
A. 2$\lambda $
B. $\lambda $
C. $\lambda $/2
D. $\lambda $/4

Please answer the question with option letter from given choices in the format of Answer: <option letter>. You FIRST think about the reasoning process as an internal monologue and then provide the final answer. The reasoning process MUST BE enclosed within <think> </think> tags. Please answer the final answer within the <answer> </answer> tags.
user
D
<image 1>A thin film with index of refraction n_1 separates two materials, each of which has an index of refraction less than nf. A monochromatic beam of light is incident normally on the film, as shown above. If the light has wavelength $\lambda $ within the film, maximum constructive interference between the incident beam and the reflected beam occurs for which of the following film thicknesses?
A. 2$\lambda $
B. $\lambda $
C. $\lambda $/2
D. $\lambda $/4

Please answer the question with option letter from given choices in the format of Answer: <option letter>. You FIRST think about the reasoning process as an internal monologue and then provide the final answer. The reasoning process MUST BE enclosed within <think> </think> tags. Please answer the final answer within the <answer> </answer> tags.
val