1. There are 4 cards (as shown in Figure 1), with one side of each card written with an English letter, and the other side written with a number. The rule is: when one side of the card is the letter $R$, its other side must be the number 2. Your task is: to check which of the following 4 cards need to be flipped to ensure there are no violations of the rule. Your choice is ( ).
(A) (a)
(B) (a), (c)
$(\mathrm{C})(\mathrm{a}),(\mathrm{d})$
(D) None of the above
$-、 1 . C$.
Thought 1: Turn over each card one by one, imagining the various possibilities on the back, which can lead to the conclusion that (b) and (c) do not need to be flipped. To aid understanding, imagine that “the red light is on” represents “one side is written with the English letter $R$”, and “pedestrians do not cross the road” represents “the other side is written with the number 2”. Checking whether the rule is violated becomes checking whether “pedestrians crossing the road when the red light is on” violates traffic rules (when the red light is not on, crossing or not crossing the road does not violate the traffic rules).

Thought 2: Understand the “rule” as: the conditional statement “if $p$ then $q$” is true, where $p$ being true represents “one side is written with the English letter $R$”, and $q$ being true represents “the other side is written with the number 2”. From the truth table 1, we can see:
(1) When $p$ is true, depending on whether $q$ is true or false, $p \rightarrow q$ can be true or false, so flipping (a) is necessary.
Table 1
\begin{tabular}{|c|c|c|}
\hline$p$ & $q$ & $p \rightarrow q$ \\
\hline True & True & True \\
\hline True & False & False \\
\hline False & True & True \\
\hline False & False & False \\
\hline
\end{tabular}
(2) When $p$ is false, regardless of whether $q$ is true or false, $p \rightarrow q$ is always true, so flipping (b) is unnecessary.
(3) When $q$ is true, regardless of whether $p$ is true or false, $p \rightarrow q$ is always true, so flipping (c) is unnecessary.
(4) When $q$ is false, depending on whether $p$ is true or false, $p \rightarrow q$ can be true or false, so flipping (d) is necessary.
Therefore, (a) and (d) should be flipped.