In the argument given, the two portions play which

[Several of a certain bank's top executives have recently been purchasing shares in their own bank. ]This activity has occasioned some surprise, since it is widely believed that the bank, carrying a large number of bad loans, is on the brink of collapse. Since the executives are well placed to know their bank's true condition, it might seem that their share purchases show that the danger of collapse is exaggerated. However, the available information about the bank's condition is from reliable and informed sources, and corporate executives do sometimes buy shares in their own company in a calculated attempt to calm worries about their company's condition. [On balance, therefore, it is likely that the executives of the bank are following this example.]
In the argument given, the two [. ]portions play which of the following roles?
A. The first describes the circumstance the explanation of which is the issue that the argument addresses; the second states the main conclusion of the argument.
B. The first describes the circumstance the explanation of which is the issue the argument addresses; the second states a conclusion that is drawn in order to support the main conclusion of the argument.
C. The first provides evidence to defend the position that the argument seeks to establish against opposing positions; the second states the main conclusion of the argument.
D. The first provides evidence to support the position that the argument seeks to establish; the second states a conclusion that is drawn in order to support the argument's main conclusion 
E. Each provides evidence to support the position that the argument seeks to establish.
The answer is A. I would appreciate it if someone explains the reason why the first is not an evidence, thanks.
Source: GWD-12-Q33