Neither eating fewer calories nor exercising more, taken separately, will guarantee that one will lose weight. To be

What we see: Critical Reasoning, "best test" question. The passage gives a rule for losing weight, and we pick the ability that would show whether a person can follow that rule.

Step 1. Pull out the rule.
What we see: "To be successful at losing weight, one must do both."
What it means: success needs two things at the same time: fewer calories and more exercise. The passage says each one alone is not enough.
What we do and why: we hold on to this two part rule, because the right test has to check both parts at once.

Step 2. See why the two parts must happen together.
What we see: more exercise can make people eat more, and fewer calories can make people too tired to exercise.
What it means: each habit tends to undo the other. So the hard part, and the thing worth testing, is keeping both going at once.
What we do and why: we look for the choice where calories go down AND exercise goes up. If exercise stays flat or drops, or calories stay flat or rise, part of the rule is missing.

Step 3. Check each choice against the two parts.
Calories down, exercise up: both parts. This is the only choice that matches.
B. Calories down, exercise down: exercise goes the wrong way.
C. Calories the same, exercise down: neither part is done.
D. Calories down, exercise the same: only one part.
E. Calories up to fuel much more exercise: calories go the wrong way.

The trap: D. Cutting calories is the classic diet advice, and keeping exercise steady sounds safe. But the passage says plainly that eating fewer calories by itself does not guarantee weight loss. D tests only one half of the rule.
Faster way: The rule is "both." Scan for the one choice where calories go down and exercise goes up. Only one choice has both moving the right way. About 15 seconds.
Takeaway: When a passage says you must do both, the right choice includes both parts, each moving the way the passage says. Any choice with only one part is the usual wrong answer.

Answer: A