Error Spotting – Choose the part (A–D) that contains the grammatical error, or choose No error if the sentence is fully correct. Sentence (parts correspond to options below): A) In order to save petrol, B) motorists must have to C) be very cautious D) while driving along the highways.

Introduction / Context:
This tests modality and the redundancy of stacking must with have to. In standard usage, must and have to both express obligation; combining them is ungrammatical or at least nonstandard in formal writing.

Given Data / Assumptions:

• Purpose clause: In order to save petrol
• Modal sequence: must have to
• Intended meaning: obligation/recommendation for motorists

Concept / Approach:
Use one marker of obligation: either must or have to. Choose must for strong speaker-imposed necessity or have to for external necessity. Do not stack them together.

Step-by-Step Solution:

Verification / Alternative check:
Substitute: They must be cautious vs They have to be cautious. Both are grammatical; the stacked form is not.

Why Other Options Are Wrong:

• A) Purpose phrase is fine.
• C) Complement be very cautious is grammatical.
• D) while + -ing clause correctly marks time/circumstance.

Common Pitfalls:
Learners often overemphasize necessity by doubling modals. Use one clear marker.

Final Answer:
Option B contains the error. Use must be or have to be, not must have to be.