I need help with ACT English problems?

<ol>
<li>He was arrested "for investigating" the February 20 robbery of the Mor-Gas Filling station at 871 Lowpoint Boulevard.</li>
</ol>

<p>A.No change
B.investigating
C.for investigation of
D.after investigating</p>

<p>I chose A, but it's C. I don't get why?</p>

<p>and also this one...</p>

<ol>
<li>The saving rate is by far the lower of any modern industrialized country and is a major factor "to" causing our high rate of inflation.</li>
</ol>

<p>E. No Change
F. in
G. on
H. for</p>

<p>I chose H, but it's F.
I was confused between those choices, but I got it wrong anyway.</p>

<p>So does it mean you never use "for [verb]ing?"</p>

<p>Can someone explain this to me?</p>