The most effective option to combine sentences 6 and 7 at the underlined portion is:C. lacks restfulness despite being healing.
Here's why:
Clarity and conciseness: This option clearly conveys the intended meaning without being overly long or wordy.
Accuracy: It accurately reflects the speaker's argument that sleep is not restorative despite having potential healing properties.
Flow and rhythm: The sentence flows naturally with the preceding and following sentences, maintaining the rhythm and coherence of the paragraph.
Here's why the other options are less effective:
A. is neither restful nor healing: This option is accurate but redundant. Since the sentence already mentions sleep's lack of restfulness, explicitly stating it is non-healing adds unnecessary information.
B. has no rest because it has no healing: This option is grammatically correct but implies a causal relationship between restfulness and healing, which might not be the intended meaning.
D. cannot be restful without being healing: This option is not accurate. The speaker argues that sleep can be healing but also lacks restfulness.
Therefore, option C best combines the two sentences while preserving the original meaning and improving the flow of the paragraph.