1. (A) it doesn't have a clear referent.
What else other than 'soil' can 'it' refer to logically? Can 'it' refer to the tortoise, meaning that tortoise is not hard for X and not soft for Y? So no doubt need be entertained about the ambiguity of the pronoun reference in A because logic is the leading light in such cases.
2. Why D is not equal to A.
D uses and infinitive as 'to make' burrowing difficult and as soft as to cause tunnels to collapse. the infinitive 'to make' implies that the
...
What else other than 'soil' can 'it' refer to logically? Can 'it' refer to the tortoise, meaning that tortoise is not hard for X and not soft for Y? So no doubt need be entertained about the ambiguity of the pronoun reference in A because logic is the leading light in such cases.
2. Why D is not equal to A.
D uses and infinitive as 'to make' burrowing difficult and as soft as to cause tunnels to collapse. the infinitive 'to make' implies that the
...
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