请教GWD23Q13

Q13:

Researchers hypothesize that granitic
soil is the ideal construction material for the desert tortoise because it is
not so hard that it makes burrowing difficult or so soft that it could cause
tunnels to collapse.

        
1. so hard that it makes burrowing difficult or so soft
        that it could cause

        
2. hard enough to make burrowing difficult or soft
        enough as to cause

        
3. so hard as to make burrowing difficult or soft
        enough so it causes

        
4. as hard as to make burrowing difficult or as soft as
        to cause

        
5. too hard, making burrowing difficult, nor too soft,
        so as to cause

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